op ['äp] -n

  1. optical art
  2. operation; operative; operator
  3. opportunity
  4. opus

OP -abbr

  1. observation post
  2. out of print

opac·i·ty [ō-'pa-sə-tē] -n

  1. obscurity of sense : unintelligibleness
  2. the quality or state of being mentally obtuse : dullness
  3. the quality or state of a body that makes it impervious to the rays of light
  4. the relative capacity of matter to obstruct the transmission of radiant energy
  5. an opaque spot in a normally transparent structure (as the lens of the eye)

opah ['ō-pə, -,pä] -n

  1. a large elliptical laterally compressed marine bony fish ( Lampris guttatusof the family Lampridae) with brilliant colors

opal ['ō-pəl] -n

  1. a usu. amorphous mineral that is a hydrated silica softer and less dense than quartz and typically with definite and often marked iridescent play of colors and is used esp. as a gem

opal·es·cent [,ō-pə-'le-sᵊnt] -adj, n, adv

  1. reflecting an iridescent light

opal·ine ['ō-pə-,līn, -,lēn] -adj

  1. resembling opal

opaque [ō-'pāk] -adj, n, adv

  1. exhibiting opacity : blocking the passage of radiant energy and esp. light
  2. hard to understand or explain
    • opaque prose
  3. obtuse, thickheaded

opaque projector -n

  1. a projector using reflected light for projecting an image of an opaque object or matter on an opaque support (as a photograph)

op art ['äp-] -n

  1. optical art

op cit -abbr

  1. in the work cited

ope ['ōp] -vb

  1. open

OPEC -abbr

  1. Organization of Petroleum Exporting Countries

oped ['äp-'ed] -n

  1. a page of special features usu. opposite the editorial page of a newspaper
  2. a feature on such a page

open ['ō-pən, -pᵊm] -adj, adv, n

  1. having no enclosing or confining barrier : accessible on all or nearly all sides
    • cattle grazing on an open range
  2. being in a position or adjustment to permit passage : not shut or locked
    • an open door
  3. having a barrier (as a door) so adjusted as to allow passage
    • the house was open
  4. having the lips parted
    • stood there with his mouth wide open
  5. not buttoned or zipped
    • an open shirt
  6. completely free from concealment : exposed to general view or knowledge
    • their hostilities eventually erupted with open war
  7. exposed or vulnerable to attack or question : subject
    • open to doubt
  8. being an operation or surgical procedure in which an incision is made such that the tissues are fully exposed
  9. not covered with a top, roof, or lid
    • an open car
    • her eyes were open
  10. having no protective covering
    • open wiring
    • an open wound
  11. : not restricted to a particular group or category of participants
    • open to the public
    • open housing
    : as
  12. enterable by both amateur and professional contestants
    • an open tournament
  13. enterable by a registered voter regardless of political affiliation
    • an open primary
  14. fit to be traveled over : presenting no obstacle to passage or view
    • the open road
    • open country
  15. having the parts or surfaces laid out in an expanded position : spread out : unfolded
    • an open book
  16. low
  17. formed with the tongue in a lower position
    • Italian has an open and a close e
  18. having clarity and resonance unimpaired by undue tension or constriction of the throat
    • an open vocal tone
  19. produced by an open string or on a wind instrument by the lip without the use of slides, valves, or keys
  20. available to follow or make use of
    • the only course open to us
  21. not taken up with duties or engagements
    • keep an hour open on Friday
  22. not finally decided : subject to further consideration
    • the salary is open
    • an open question
  23. available for a qualified applicant : vacant
    • the job is still open
  24. remaining available for use or filling until canceled
    • an open order for more items
  25. available for future purchase
    • these items are in open stock
  26. available for breeding : not now pregnant
    • an open heifer
  27. not proprietary : available to third party developers
    • open source code
  28. : characterized by ready accessibility and usu. generous attitude: as
  29. generous in giving
  30. willing to hear and consider or to accept and deal with : responsive
  31. free from reserve or pretense : frank
  32. accessible to the influx of new factors (as foreign goods)
    • an open market
  33. : having openings, interruptions, or spaces
    • open mesh
    : as
  34. being porous and friable
    • open soil
  35. sparsely distributed : scattered
    • open population
  36. having components separated by a space in writing or printing (as opaque projector)
  37. not made up of a continuous closed circuit of channels
    • the insect circulatory system is open
  38. not stopped at the top
  39. not stopped by the finger
  40. being in operation
    • an open microphone
  41. ready for business, patronage, or use
    • the store is open from 9 to 5
    • the new highway will be open next week
  42. characterized by lack of effective regulation of various commercial enterprises
    • an open town
  43. not repressed by legal controls
    • open gambling
  44. free from checking or hampering restraints
    • an open economy
  45. relatively unguarded by opponents
    • passed to an open teammate
  46. having been opened by a first ante, bet, or bid
    • the bidding is open
  47. characterized by sparing use esp. of the comma
  48. containing none of its endpoints
    • an open interval
  49. being a set or composed of sets each point of which has a neighborhood all of whose points are contained in the set
    • the interior of a sphere is an open set
  50. being an incomplete electrical circuit
  51. not allowing the flow of electricity
    • an open switch
  52. having insufficient mass to halt expansion gravitationally
  53. to move (as a door) from a closed position
  54. to make available for entry or passage by turning back (as a barrier) or removing (as a cover or an obstruction)
  55. to make available for or active in a regular function
    • open a new store
  56. to make accessible for a particular purpose
    • open ed new land for settlement
  57. to initiate access to (a computer file) prior to use
  58. to disclose or expose to view : reveal
  59. to make more discerning or responsive : enlighten
    • must open our minds to the problems
  60. to bring into view or come in sight of by changing position
  61. to make an opening in
    • open ed the boil
  62. to loosen and make less compact
    • open the soil
  63. to spread out : unfold
    • open ed the book
  64. to enter upon : begin
    • open ed the meeting
  65. to commence action in a card game by making (a first bid), putting a first bet in (the pot), or playing (a card or suit) as first lead
  66. to restore or recall (as an order) from a finally determined state to a state in which the parties are free to prosecute or oppose
  67. to become open
    • the office open ed early
  68. to spread out : expand
    • the wound open ed under the strain
  69. to become disclosed
    • a beautiful vista open ed before us
  70. to become enlightened or responsive
  71. to give access
    • the rooms open onto a hall
  72. speak out
    • finally he open ed freely on the subject
  73. to begin a course or activity
  74. to make a bet, bid, or lead in commencing a round or hand of a card game
  75. to provide the opening performance of a show before the main event
  76. opening
  77. : open and unobstructed space: as
  78. open air
  79. open water
  80. an open contest, competition, or tournament
  81. a public or unconcealed state or position

open admission -n

  1. open enrollment

open adoption -n

  1. an adoption that involves contact between biological and adoptive parents and sometimes between biological parents and the adopted child

openair -adj

  1. outdoor

open air -n

  1. the space where air is unconfined
  2. outdoors

openandshut -adj

  1. perfectly simple : obvious
  2. easily settled
    • an openandshut case of grand larceny

open arms -n pl

  1. an eager or warm welcome
    • greeted them with open arms

open bar -n

  1. a bar (as at a wedding reception) at which drinks are served free

open·cast ['ō-pən-,kast] -adj

  1. worked from a surface open to the air
    • an opencast mine
    • opencast mining

open chain -n

  1. an arrangement of atoms represented in a structural formula by a chain whose ends are not joined so as to form a ring

open city -n

  1. a city that is not occupied or defended by military forces and that is not allowed to be bombed under international law

open dating -n

  1. the marking of perishable food products with a clearly readable date indicating when the food was packaged or the last date on which it should be sold or used

open door -n, adj

  1. a recognized right of admittance : freedom of access
  2. a policy providing such freedom
  3. a policy giving opportunity for commercial relations with a country to all nations on equal terms

openend -adj

  1. : organized to allow for contingencies: as
  2. permitting additional debt to be incurred under the original indenture subject to specified conditions
    • an openend mortgage
  3. having a fluctuating capitalization of shares that are issued or redeemed at the current net asset value or at a figure in fixed ratio to this
    • an openend investment company

openend·ed [,ō-pən-'en-dəd] -adj, n

  1. : not rigorously fixed: as
  2. adaptable to the developing needs of a situation
  3. permitting or designed to permit spontaneous and unguided responses

open enrollment -n

  1. the voluntary enrollment of a student in a public school other than the one assigned on the basis of residence
  2. enrollment on demand as a student in an institution of higher learning irrespective of formal qualifications

open·er ['ōp-nər, 'ō-pə-] -n

  1. : one that opens
    • a bottle opener
    : as
  2. cards of sufficient value for a player to open the betting in a poker game
  3. the first item, contest, or event of a series

openeyed [,ō-pən-'īd] -adj

  1. having the eyes open
  2. carefully observant : discerning

openfaced [-'fāst] -adj

  1. served without a covering layer (as of bread or pastry)
    • an openfaced sandwich

open·hand·ed [-'han-dəd] -adj, adv, n

  1. generous, munificent

openheart -adj

  1. of, relating to, or performed on a heart temporarily stopped and relieved of circulatory function and surgically opened for repair of defects or damage
    • openheart surgery

open·heart·ed [,ō-pən-'här-təd] -adj, adv, n

  1. candidly straightforward : frank
  2. responsive to emotional appeal

openhearth -adj

  1. of, relating to, involving, or produced in the open-hearth process
    • openhearth steel

openhearth process -n

  1. a process of making steel from pig iron in a reverberatory furnace equipped with a regenerator

open house -n

  1. ready and usu. informal hospitality or entertainment for all comers
  2. a house or apartment open for inspection esp. by prospective buyers or tenants

open·ing ['ōp-niŋ, 'ō-pə-] -n

  1. an act or instance of making or becoming open
  2. an act or instance of beginning : commencement
  3. a formal and usu. public event by which something new is put officially into operation
  4. : something that is open: as
  5. breach, aperture
  6. an open width : span
  7. an area without trees or with scattered usu. mature trees that occurs as a break in a forest
  8. two pages that face one another in a book
  9. : something that constitutes a beginning: as
  10. a usu. planned series of moves made at the beginning of a game of chess or checkers
  11. a first performance
  12. occasion, chance
  13. an opportunity for employment

openlabel -adj

  1. being or relating to a clinical trial in which the treatment given to each subject is not concealed from either the experimenters or the subject
    • an openlabel multicenter study

open letter -n

  1. a published letter of protest or appeal usu. addressed to an individual but intended for the general public

open loop -n

  1. a control system for an operation or process in which there is no self-correcting action as there is in a closed loop

open marriage -n

  1. a marriage in which the partners agree to let each other have sexual partners outside the marriage

open mike -n

  1. an event in which amateurs may perform (as at a coffeehouse) usu. without auditioning first

openmind·ed [,ō-pən-'mīn-dəd] -adj, adv, n

  1. receptive to arguments or ideas

open·mouthed [,ō-pən-'mau̇thd, -'mau̇tht] -adj, adv, n

  1. clamorous, vociferous
  2. having the mouth wide open
  3. struck with amazement or wonder

openpol·li·nat·ed [,ō-pən-'pä-lə-,nā-təd] -adj

  1. pollinated by natural agencies (as wind or insects) without human intervention

open season -n

  1. a period when it is legal to kill or catch game or fish protected at other times by law
  2. a time during which someone or something is the object of sustained attack or criticism

open secret -n

  1. a supposedly secret but generally known matter

open sentence -n

  1. a statement (as in mathematics) that contains at least one blank or unknown and that becomes true or false when the blank is filled or a quantity is substituted for the unknown

open ses·a·me [-'se-sə-mē] -n

  1. something that unfailingly brings about a desired end

open shop -n

  1. an establishment in which eligibility for employment and retention on the payroll are not determined by membership or nonmembership in a labor union though there may be an agreement by which a union is recognized as sole bargaining agent

open sight -n

  1. a firearm rear sight having an open notch

open stance -n

  1. a stance (as in golf) in which the forward foot is farther from the line of play than the back foot

open syllable -n

  1. a syllable ended by a vowel or diphthong

open up -vt

  1. to make available
  2. to make plain or visible : disclose
  3. to open by cutting into
  4. to spread out or come into view
    • the road opens up ahead
  5. to commence firing
  6. to become communicative
    • tried to get the patient to open up

open·work ['ō-pən-,wərk] -n, adj

  1. work constructed so as to show openings through its substance : work that is perforated or pierced
    • wrought-iron openwork

opera ['ō-pəs] -n

(variant of opus)

  1. work
  2. a musical composition or set of compositions usu. numbered in the order of its issue

op·era ['ä-p(ə-)rə, Southern also 'ä-prē] -n

  1. a drama set to music and made up of vocal pieces with orchestral accompaniment and orchestral overtures and interludes
  2. grand opera
  3. the score of a musical drama
  4. the performance of an opera
  5. a house where operas are performed

op·er·a·ble ['ä-p(ə-)rə-bəl] -adj, n, adv

  1. fit, possible, or desirable to use : practicable
  2. likely to result in a favorable outcome upon surgical treatment
    • an operable cancer

op·era bouffe ['ō-pā-rä-'büf, 'ä-p(ə-)rə-] -n

  1. satirical comic opera

opera buf·fa [-'bü-fə] -n

  1. an 18th century farcical comic opera with dialogue in recitative

opera co·mique [-kä-'mēk, -kō-] -n

  1. an opera characterized by spoken dialogue interspersed between the arias and ensemble numbers

opera glass -n

  1. a small low-power binocular without prisms for use at the opera or theater often used in pl.

op·era·go·er ['ä-p(ə-)rə-,gō-ər] -n

  1. a person who frequently goes to operas

opera hat -n

  1. a man's collapsible top hat

opera house -n

  1. a theater devoted principally to the performance of operas
  2. theater

op·er·and [,ä-pə-'rand] -n

  1. something (as a quantity or data) that is operated on (as in a mathematical operation)
  2. the address in a computer instruction of data to be operated on

op·er·ant ['ä-pə-rənt] -adj, adv

  1. functioning or tending to produce effects : effective
    • an operant conscience
  2. of or relating to the observable or measurable
  3. of, relating to, or being an operant or operant conditioning
    • operant behavior

operant -n

  1. behavior (as bar pressing by a rat to obtain food) that operates on the environment to produce rewarding and reinforcing effects

operant conditioning -n

  1. conditioning in which the desired behavior or increasingly closer approximations to it are followed by a rewarding or reinforcing stimulus

opera se·ria [-'ser-ē-ə, -'sir-] -n

  1. an 18th century opera with a heroic or legendary subject

op·er·ate ['ä-pə-,rāt, 'ä-,prāt] -vb

  1. to perform a function : exert power or influence
    • factors operating against our success
  2. to produce an appropriate effect
    • the drug operated quickly
  3. to perform an operation or a series of operations
  4. to perform surgery
  5. to carry on a military or naval action or mission
  6. to follow a course of conduct that is often irregular
    • crooked gamblers operating in the club
  7. bring about, effect
  8. to cause to function : work
  9. to put or keep in operation
  10. to perform an operation on
  11. to perform surgery on

op·er·at·ic [,ä-pə-'ra-tik] -adj, adv

  1. of or relating to opera
  2. grand, dramatic, or romantic in style or effect

operating -adj

  1. of, relating to, or used for or in operations
    • operating expenses
    • a hospital operating room

operating system -n

  1. software that controls the operation of a computer and directs the processing of programs (as by assigning storage space in memory and controlling input and output functions)

op·er·a·tion [,ä-pə-'rā-shən] -n

  1. performance of a practical work or of something involving the practical application of principles or processes
  2. an exertion of power or influence
    • the operation of a drug
  3. the quality or state of being functional or operative
    • the plant is now in operation
  4. a method or manner of functioning
    • a machine of very simple operation
  5. efficacy, potency archaic except in legal usage
  6. a procedure performed on a living body usu. with instruments esp. for the repair of damage or the restoration of health
  7. any of various mathematical or logical processes (as addition) of deriving one entity from others according to a rule
  8. a usu. military action, mission, or maneuver including its planning and execution
  9. the office on the flight line of an airfield where pilots file clearance for flights and where flying from the field is controlled
  10. the agency of an organization charged with carrying on the principal planning and operating functions of a headquarters and its subordinate units
  11. a business transaction esp. when speculative
  12. a single step performed by a computer in the execution of a program
  13. a usu. small business or establishment
    • ran a struggling operation

op·er·a·tion·al [-shnəl, -shə-nᵊl] -adj, adv

  1. of or relating to operation or to an operation
    • the operational gap between planning and production
  2. of, relating to, or based on operations
  3. of, engaged in, or connected with execution of military or naval operations in campaign or battle
  4. ready for or in condition to undertake a destined function

op·er·a·tion·al·ism [-shnə-,li-zəm, -shə-nə-,li-zəm] -n, adj

  1. a view that the concepts or terms used in nonanalytic scientific statements must be definable in terms of identifiable and repeatable operations

op·er·a·tion·ism [,ä-pə-'rā-shə-,ni-zəm] -n

  1. operationalism

operations research -n

  1. the application of scientific and esp. mathematical methods to the study and analysis of problems involving complex systems called also operational research

op·er·a·tive ['ä-p(ə-)rə-tiv, 'ä-pə-,rā-] -adj, adv, n

  1. producing an appropriate effect : efficacious
  2. most significant or essential
    • the operative word in a phrase
  3. exerting force or influence : operating
  4. having to do with physical operations (as of machines)
  5. working
    • an operative craftsman
  6. based on or consisting of an operation
    • operative dentistry

operative -n

  1. : operator: as
  2. artisan, mechanic
  3. a secret agent
  4. private detective
  5. a person who works toward achieving the objectives of a larger interest
    • political operative s

op·er·a·tor ['ä-pə-,rā-tər, 'ä-,prā-] -n, adj

  1. : one that operates: as
  2. one that operates a machine or device
  3. one that operates a business
  4. one that performs surgical operations
  5. one that deals in stocks or commodities
  6. mountebank, fraud
  7. a shrewd and skillful person who knows how to circumvent restrictions or difficulties
  8. something and esp. a symbol that denotes or performs a mathematical or logical operation
  9. a mathematical function
  10. a binding site in a DNA chain at which a genetic repressor binds to inhibit the initiation of transcription of messenger RNA by one or more nearby structural genes called also operator gene

oper·cu·lar [ō-'pər-kyə-lər] -adj

  1. of, relating to, or constituting an operculum

opercular -n

  1. an opercular part (as a bone or scale)

oper·cu·late [ō-'pər-kyə-lət] -adj

  1. having an operculum

oper·cu·lum [ō-'pər-kyə-ləm] -n

  1. : a body process or part that suggests a lid: as
  2. a horny or shelly plate on the posterior dorsal surface of the foot in many gastropod mollusks that closes the shell when the animal is retracted
  3. the covering of the gills of a fish
  4. a lid or covering flap (as of a moss capsule)

op·er·et·ta [,ä-pə-'re-tə] -n

  1. a usu. romantic comic opera that includes songs and dancing

op·er·on ['ä-pə-,rän] -n

  1. a group of closely linked genes that produces a single messenger RNA molecule in transcription and that consists of structural genes and regulating elements (as an operator and promoter)

op·er·ose ['ä-pə-,rōs] -adj, adv, n

  1. tedious, wearisome

Ophe·lia [ō-'fēl-yə] -n

  1. the daughter of Polonius in Shakespeare's Hamlet

ophid·i·an [ō-'fi-dē-ən] -adj, n

  1. of, relating to, or resembling snakes

Ophir ['ō-fər] -n

  1. a biblical land of uncertain location but reputedly rich in gold

ophit·ic [ä-'fi-tik, ō-] -adj

  1. having or being a rock fabric in which lath-shaped plagioclase crystals are enclosed in later formed augite

ophi·u·roid [,ō-fē-'yu̇r-,ȯid, ,ä-] -n, adj

  1. brittle star

ophthalm- -comb form

  1. eye
    • ophthalmo logy

oph·thal·mia [äf-'thal-mē-ə, äp-] -n

  1. inflammation of the conjunctiva or the eyeball

oph·thal·mic [-mik] -adj

  1. of, relating to, or situated near the eye
  2. supplying or draining the eye or structures in the region of the eye
    • ophthalmic artery

oph·thal·mol·o·gist [,äf-thə(l)-'mä-lə-jist, ,äp-, -,thal-] -n

  1. a physician who specializes in ophthalmology

oph·thal·mol·o·gy [-'mä-lə-jē] -n, adj, adv

  1. a branch of medical science dealing with the structure, functions, and diseases of the eye

oph·thal·mo·scope [äf-'thal-mə-,skōp, äp-] -n, adj

  1. an instrument for use in viewing the interior of the eye and esp. the retina

opi·ate ['ō-pē-ət, -,āt] -n

  1. a drug (as morphine or codeine) containing or derived from opium and tending to induce sleep and alleviate pain
  2. narcotic
  3. opioid
  4. something that induces rest or inaction or quiets uneasiness

opiate -adj

  1. containing or mixed with opium
  2. of, relating to, binding, or being an opiate
    • opiate receptors
  3. inducing sleep : narcotic
  4. causing dullness or inaction

opine [ō-'pīn] -vb

  1. to express opinions
  2. to state as an opinion

opin·ion [ə-'pin-yən] -n, adj

  1. a view, judgment, or appraisal formed in the mind about a particular matter
  2. approval, esteem
  3. belief stronger than impression and less strong than positive knowledge
  4. a generally held view
  5. a formal expression of judgment or advice by an expert
  6. the formal expression (as by a judge, court, or referee) of the legal reasons and principles upon which a legal decision is based

opin·ion·at·ed [-yə-,nā-təd] -adj, adv, n

  1. unduly adhering to one's own opinion or to preconceived notions

opin·ion·a·tive [-,nā-tiv] -adj, adv, n

  1. of, relating to, or consisting of opinion : doctrinal
  2. opinionated

opi·oid ['ō-pē-,ȯid] -adj

  1. possessing some properties characteristic of opiate narcotics but not derived from opium
  2. of, involving, or induced by an opioid

opioid -n

  1. any of a group of endogenous neural polypeptides (as an endorphin or enkephalin) that bind esp. to opiate receptors and mimic some of the pharmacological properties of opiates called also opioid peptide
  2. a synthetic drug possessing narcotic properties similar to opiates but not derived from opium
  3. opiate

opis·tho·branch [ə-'pis-thə-,braŋk] -n, adj

  1. any of a subclass (Opisthobranchia) of marine gastropod mollusks that have the gills when present posterior to the heart and often lack a shell

opi·um ['ō-pē-əm] -n

  1. a bitter brownish addictive narcotic drug that consists of the dried latex obtained from immature seed capsules of the opium poppy
  2. something having an effect like that of opium

opium poppy -n

  1. an annual Eurasian poppy ( Papaver somniferum) cultivated since antiquity as the source of opium, for its edible oily seeds, or for its showy flowers

opos·sum [(ə-)'pä-səm] -n

  1. any of a family (Didelphidae) of American marsupials that usu. have a pointed snout and prehensile tail
  2. a common omnivorous largely nocturnal mammal ( Didelphis virginiana) of No. America that has grayish to blackish fur with white on the cheeks and is an expert climber
  3. any of several Australian phalangers

opp -abbr

  1. opposite

op·po·nent [ə-'pō-nənt] -n

  1. one that takes an opposite position (as in a debate, contest, or conflict)
  2. a muscle that opposes or counteracts and limits the action of another

opponent -adj

  1. antagonistic, opposing
  2. situated in front

op·por·tune [,ä-pər-'tün, -'tyün] -adj, adv, n

  1. suitable or convenient for a particular occurrence
    • an opportune moment
  2. occurring at an appropriate time
    • an opportune offer of assistance

op·por·tun·ism [-'tü-,ni-zəm, -'tyü-] -n

  1. the art, policy, or practice of taking advantage of opportunities or circumstances often with little regard for principles or consequences

op·por·tun·ist [-'tü-nist, -'tyü-] -n, adj

  1. one that is opportunistic or that practices opportunism

op·por·tu·nis·tic [-tü-'nis-tik, -tyü-] -adj, adv

  1. : taking advantage of opportunities as they arise: as
  2. exploiting opportunities with little regard to principle or consequences
    • a politician considered opportunistic
  3. feeding on whatever food is available
    • opportunistic feeders
  4. being or caused by a usu. harmless microorganism that can become pathogenic when the host's resistance is impaired
    • opportunistic infections

op·por·tu·ni·ty [,ä-pər-'tü-nə-tē, -'tyü-] -n

  1. a favorable juncture of circumstances
    • the halt provided an opportunity for rest and refreshment
  2. a good chance for advancement or progress

opportunity cost -n

  1. the added cost of using resources (as for production or speculative investment) that is the difference between the actual value resulting from such use and that of an alternative (as another use of the same resources or an investment of equal risk but greater return)

op·pos·able [ə-'pō-zə-bəl] -adj, n

  1. capable of being opposed or resisted
  2. capable of being placed against one or more of the remaining digits of a hand or foot
    • the opposable human thumb

op·pose [ə-'pōz] -vt, n

  1. to place over against something so as to provide resistance, counterbalance, or contrast
  2. to place opposite or against something
  3. to offer resistance to

op·posed [-'pōzd] -adj

  1. set or placed in opposition : contrary
    • with politicians, as opposed to soap, you cannot return what you have bought -- Felix G. Rohatyn

op·pose·less [ə-'pōz-ləs] -adj

  1. irresistible

op·po·site ['ä-pə-zət, 'äp-sət] -adj, adv, n

  1. set over against something that is at the other end or side of an intervening line or space
    • opposite interior angles
    • opposite ends of a diameter
  2. situated in pairs on an axis with each member being separated from the other by half the circumference of the axis
    • opposite leaves
  3. occupying an opposing and often antagonistic position
    • opposite sides of the question
  4. diametrically different (as in nature or character)
    • opposite meanings
  5. contrary to one another or to a thing specified : reverse
    • gave them opposite directions
  6. being the other of a pair that are corresponding or complementary in position, function, or nature
    • members of the opposite sex
  7. of, relating to, or being the side of a baseball field that is near the first base line for a right-handed batter and near the third base line for a left-handed batter

opposite -n

  1. something that is opposed to some other often specified thing
  2. antonym
  3. additive inverse
  4. the additive inverse of a real number
  5. on or to an opposite side
  6. across from and usu. facing or on the same level with
    • sat opposite each other
  7. in a role complementary to
    • played opposite the leading man in the comedy

opposite number -n

  1. a member of a system or class who holds relatively the same position as a particular member in a corresponding system or class : counterpart

op·po·si·tion [,ä-pə-'zi-shən] -n, adj

  1. a configuration in which one celestial body is opposite another (as the sun) in the sky or in which the elongation is near or equal to 180 degrees
  2. the relation between two propositions having the same subject and predicate but differing in quantity or quality or both
  3. an act of setting opposite or over against : the condition of being so set
  4. hostile or contrary action or condition
  5. something that opposes
  6. a body of persons opposing something
  7. a political party opposing and prepared to replace the party in power

op·po·si·tion·ist [-'zi-sh(ə-)nist] -n, adj

  1. a member of an opposition

op·press [ə-'pres] -vt, n

  1. suppress
  2. to crush or burden by abuse of power or authority
  3. to burden spiritually or mentally : weigh heavily upon

op·pres·sion [ə-'pre-shən] -n

  1. unjust or cruel exercise of authority or power
  2. something that oppresses esp. in being an unjust or excessive exercise of power
  3. a sense of being weighed down in body or mind : depression

op·pres·sive [ə-'pre-siv] -adj, adv, n

  1. unreasonably burdensome or severe
    • oppressive legislation
  2. tyrannical
  3. overwhelming or depressing to the spirit or senses
    • an oppressive climate

op·pro·bri·ous [ə-'prō-brē-əs] -adj, adv, n

  1. expressive of opprobrium : scurrilous
    • opprobrious language
  2. deserving of opprobrium : infamous

op·pro·bri·um [-brē-əm] -n

  1. something that brings disgrace
  2. public disgrace or ill fame that follows from conduct considered grossly wrong or vicious
  3. contempt, reproach

op·pugn [ə-'pyün, ä-] -vt, n

  1. to fight against
  2. to call in question

Ops ['äps] -n

  1. the Roman goddess of abundance and the wife of Saturn

op·sin ['äp-sən] -n

  1. any of various colorless proteins that in combination with retinal or a related prosthetic group form a visual pigment (as rhodopsin) in a reaction reversible by light

op·son·ic [äp-'sä-nik] -adj

  1. of, relating to, or involving opsonin

op·so·nin ['äp-sə-nən] -n

  1. any of various proteins (as antibodies or complement) that bind to foreign particles and cells (as bacteria) making them more susceptible to the action of phagocytes

opt ['äpt] -vi

  1. to make a choice
  2. to decide in favor of something
    • opt ed for a tax increase -- Tom Wicker
  3. optical; optician; optics
  4. option; optional

op·ta·tive ['äp-tə-tiv] -adj, n, adv

  1. of, relating to, or constituting a verbal mood that is expressive of wish or desire
  2. of, relating to, or constituting a sentence that is expressive of wish or hope
  3. expressing desire or wish

op·tic ['äp-tik] -adj

  1. of or relating to vision or the eye

optic -n

  1. eye
  2. any of the elements (as lenses, mirrors, or light guides) of an optical instrument or system usu. used in pl.
  3. an optical instrument

op·ti·cal ['äp-ti-kəl] -adj, adv

  1. of or relating to the science of optics
  2. of or relating to vision : visual
  3. visible
    • optical wavelength
  4. of, relating to, or being objects that emit light in the visible range of frequencies
    • an optical galaxy
  5. using the properties of light to aid vision
    • an optical instrument
  6. of, relating to, or utilizing light esp. instead of other forms of energy
    • optical microscopy
  7. involving the use of light-sensitive devices to acquire information for a computer
    • optical character recognition
  8. of or relating to optical art

optical activity -n

  1. ability of a chemical substance to rotate the plane of vibration of polarized light to the right or left

optical art -n

  1. nonobjective art characterized by the use of straight or curved lines or geometric patterns often for an illusory effect (as of motion)

optical bench -n

  1. an apparatus that is fitted for the convenient location and adjustment of light sources and optical devices and that is used for the observation and measurement of optical phenomena

optical disk -n

  1. a disk with a plastic coating on which information (as music, visual images, or computer data) is recorded digitally (as in the form of tiny pits) and which is read by using a laser

optical fiber -n

  1. a single fiber-optic strand

optical glass -n

  1. flint or crown glass of well-defined characteristics used esp. for making lenses

optical illusion -n

  1. illusion

optically active -adj

  1. capable of rotating the plane of vibration of polarized light to the right or left used of compounds, molecules, or atoms

optical rotation -n

  1. the angle through which the plane of vibration of polarized light that traverses an optically active substance is rotated

optic axis -n

  1. a line in a doubly refracting medium that is parallel to the direction in which all components of plane-polarized light travel with the same speed

optic chiasma -n

  1. the X-shaped partial decussation on the undersurface of the hypothalamus through which the optic nerves are continuous with the brain called also optic chiasm

optic cup -n

  1. the optic vesicle after invaginating to form a 2-layered cup from which the retina and pigmented layer of the eye will develop called also eyecup

optic disk -n

  1. blind spot

op·ti·cian [äp-'ti-shən] -n

  1. a maker of or dealer in optical items and instruments
  2. a person who reads prescriptions for visual correction, orders lenses, and dispenses eyeglasses and contact lenses

optic lobe -n

  1. either of two prominences of the midbrain concerned with vision

optic nerve -n

  1. either of the second pair of cranial nerves that pass from the retina to the optic chiasma and conduct visual stimuli to the brain

op·tics ['äp-tiks] -n pl but sing in constr

  1. a science that deals with the genesis and propagation of light, the changes that it undergoes and produces, and other phenomena closely associated with it

optic vesicle -n

  1. an evagination of each lateral wall of the embryonic vertebrate forebrain from which the nervous structures of the eye develop

op·ti·mal ['äp-tə-məl] -adj, n, adv

  1. most desirable or satisfactory : optimum

op·ti·mi·sa·tion [,äp-tə-mə-'zā-shən] -n

(variant of optimization)

  1. an act, process, or methodology of making something (as a design, system, or decision) as fully perfect, functional, or effective as possible
  2. the mathematical procedures (as finding the maximum of a function) involved in this

op·ti·mism ['äp-tə-,mi-zəm] -n, adj, adv

  1. a doctrine that this world is the best possible world
  2. an inclination to put the most favorable construction upon actions and events or to anticipate the best possible outcome

Op·ti·mist ['äp-tə-mist] -n

  1. a member of a major international service club

op·ti·mi·za·tion [,äp-tə-mə-'zā-shən] -n

  1. an act, process, or methodology of making something (as a design, system, or decision) as fully perfect, functional, or effective as possible
  2. the mathematical procedures (as finding the maximum of a function) involved in this

op·ti·mize ['äp-tə-,mīz] -vt, n

  1. to make as perfect, effective, or functional as possible

op·ti·mum ['äp-tə-məm] -n, adj

  1. the amount or degree of something that is most favorable to some end
  2. the most favorable condition for the growth and reproduction of an organism
  3. greatest degree attained or attainable under implied or specified conditions

op·tion ['äp-shən] -n

  1. an act of choosing
  2. the power or right to choose : freedom of choice
  3. a privilege of demanding fulfillment of a contract on any day within a specified time
  4. a contract conveying a right to buy or sell designated securities, commodities, or property interest at a specified price during a stipulated period
  5. the right conveyed by an option
  6. a right of an insured person to choose the form in which payments due on a policy shall be made or applied
  7. : something that may be chosen: as
  8. an alternative course of action
    • didn't have many option s open
  9. an item that is offered in addition to or in place of standard equipment
  10. an offensive football play in which a back may choose whether to pass or run with the ball called also option play

option -vt

  1. to grant or take an option on
  2. to acquire the exclusive right to use (an author's work) as the basis for a motion picture
    • the studio option ed the novel for a film

op·tion·al ['äp-shnəl, -shə-nᵊl] -adj, n, adv

  1. involving an option : not compulsory

opto- -comb form

  1. vision
    • opto metry
  2. optic and
    • opto electronics

op·to·elec·tron·ics [,äp-(,)tō-i-lek-'trä-niks, -ē-lek-] -n pl but sing in constr, adj

  1. a branch of electronics that deals with electronic devices for emitting, modulating, transmitting, and sensing light

op·to·ki·net·ic [,äp-tō-kə-'ne-tik, -kī-] -adj

  1. of, relating to, or involving movements of the eyes

op·tom·e·trist [äp-'tä-mə-trist] -n

  1. a specialist licensed to practice optometry

op·tom·e·try [-trē] -n, adj

  1. the health-care profession concerned esp. with examining the eye for defects and faults of refraction, with prescribing correctional lenses or eye exercises, with diagnosing diseases of the eye, and with treating such diseases or referring them for treatment

opt out -vi

  1. to choose not to participate in something often used with of
    • opted out of the project

op·u·lence ['ä-pyə-lən(t)s] -n

  1. wealth, affluence
  2. abundance, profusion

op·u·lent [-lənt] -adj, adv

  1. : exhibiting or characterized by opulence: as
  2. having a large estate or property : wealthy
    • hoping to marry an opulent widow
  3. amply or plentifully provided or fashioned often to the point of ostentation
    • living in opulent comfort

opun·tia [ō-'pən(t)-sh(ē-)ə] -n

  1. any of a large genus ( Opuntia) of American cacti with usu. yellow flowers and flat or terete joints usu. studded with tubercles bearing spines or prickly hairs

opus ['ō-pəs] -n

  1. work
  2. a musical composition or set of compositions usu. numbered in the order of its issue

opus·cule [ō-'pəs-(,)kyül] -n

  1. a small or petty work : opusculum

opus·cu·lum [ō-'pəs-kyə-ləm] -n

  1. a minor work (as of literature) usu. used in pl.