| opacification | (noun) the process of becoming cloudy or opaque | - |
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| opacity | (noun) the quality of being opaque to a degree; the degree to which something reduces the passage of light | Synonyms: opaqueness |
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| (noun) incomprehensibility resulting from obscurity of meaning | Synonyms: opaqueness |
| (noun) the phenomenon of not permitting the passage of electromagnetic radiation | - |
| opah | (noun) large elliptical brightly colored deep-sea fish of Atlantic and Pacific and Mediterranean | Synonyms: Lampris regius, moonfish |
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| opal | (noun) a translucent mineral consisting of hydrated silica of variable color; some varieties are used as gemstones | - |
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| opalescence | (noun) the visual property of something having a milky brightness and a play of colors from the surface | Synonyms: iridescence |
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| opaqueness | (noun) the quality of being opaque to a degree; the degree to which something reduces the passage of light | Synonyms: opacity |
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| (noun) incomprehensibility resulting from obscurity of meaning | Synonyms: opacity |
| open | (noun) information that has become public | Synonyms: surface |
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| (noun) a tournament in which both professionals and amateurs may play | - |
| (noun) a clear or unobstructed space or expanse of land or water | Synonyms: clear |
| (noun) where the air is unconfined | Synonyms: open air, out-of-doors, outdoors |
| openbill | (noun) stork with a grooved bill whose upper and lower parts touch only at the base and tip | - |
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| opener | (noun) a hand tool used for opening sealed containers (bottles or cans) | - |
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| (noun) the first event in a series | - |
| (noun) a person who unfastens or unwraps or opens | Synonyms: undoer, unfastener, untier |
| openhandedness | (noun) liberality in bestowing gifts; extremely liberal and generous of spirit | Synonyms: largess, largesse, magnanimity, munificence |
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| opening | (noun) a recognized sequence of moves at the beginning of a game of chess | Synonyms: chess opening |
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| (noun) the act of opening something | - |
| (noun) becoming open or being made open | - |
| (noun) the first of a series of actions | Synonyms: first step, initiative, opening move |
| (noun) a vacant or unobstructed space that is man-made | - |
| (noun) an entrance equipped with a hatch; especially a passageway between decks of a ship | Synonyms: hatchway, scuttle |
| (noun) an aperture or hole that opens into a bodily cavity | Synonyms: orifice, porta |
| (noun) a possible alternative | Synonyms: possibility, possible action |
| (noun) the initial part of the introduction | - |
| (noun) the first performance (as of a theatrical production) | Synonyms: curtain raising, opening night |
| (noun) a ceremony accompanying the start of some enterprise | - |
| (noun) an open or empty space in or between things | Synonyms: gap |
| (noun) opportunity especially for employment or promotion | - |
| openness | (noun) characterized by an attitude of ready accessibility (especially about one's actions or purposes); without concealment; not secretive | Synonyms: nakedness |
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| (noun) willingness or readiness to receive (especially impressions or ideas) | Synonyms: receptiveness, receptivity |
| (noun) without obstructions to passage or view | - |
| openwork | (noun) ornamental work (such as embroidery or latticework) having a pattern of openings | - |
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| opepe | (noun) large African forest tree yielding a strong hard yellow to golden brown lumber; sometimes placed in genus Sarcocephalus | Synonyms: Nauclea diderrichii, Sarcocephalus diderrichii |
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| opera | (noun) a building where musical dramas are performed | Synonyms: opera house |
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| (noun) a drama set to music; consists of singing with orchestral accompaniment and an orchestral overture and interludes | - |
| operagoer | (noun) a patron of the opera | - |
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| operand | (noun) a quantity upon which a mathematical operation is performed | - |
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| operation | (noun) a medical procedure involving an incision with instruments; performed to repair damage or arrest disease in a living body | Synonyms: surgery, surgical operation, surgical procedure, surgical process |
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| (noun) (mathematics) calculation by mathematical methods | Synonyms: mathematical operation, mathematical process |
| (noun) activity by a military or naval force (as a maneuver or campaign) | Synonyms: military operation |
| (noun) a planned activity involving many people performing various actions | - |
| (noun) a process or series of acts especially of a practical or mechanical nature involved in a particular form of work | Synonyms: procedure |
| (noun) a business especially one run on a large scale | - |
| (noun) the activity of operating something (a machine or business etc.) | - |
| (noun) (psychology) the performance of some composite cognitive activity; an operation that affects mental contents | Synonyms: cognitive operation, cognitive process, mental process, process |
| (noun) (computer science) data processing in which the result is completely specified by a rule (especially the processing that results from a single instruction) | - |
| (noun) process or manner of functioning or operating | Synonyms: functioning, performance |
| (noun) the state of being in effect or being operative | - |
| operationalism | (noun) (philosophy) the doctrine that the meaning of a proposition consists of the operations involved in proving or applying it | - |
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| operations | (noun) financial transactions at a brokerage; having to do with the execution of trades and keeping customer records | Synonyms: trading operations |
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| operative | (noun) someone who can be employed as a detective to collect information and assist in criminal investigations | Synonyms: PI, private detective, private eye, private investigator, shamus, sherlock |
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| (noun) a person secretly employed in espionage for a government | Synonyms: intelligence agent, intelligence officer, secret agent |
| operator | (noun) an agent that operates some apparatus or machine | Synonyms: manipulator |
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| (noun) someone who owns or operates a business | - |
| (noun) a shrewd or unscrupulous person who knows how to circumvent difficulties | Synonyms: hustler, wheeler dealer |
| (noun) a speculator who trades aggressively on stock or commodity markets | - |
| (noun) (mathematics) a symbol or function representing a mathematical operation | - |
| operculum | (noun) a hard flap serving as a cover for (a) the gill slits in fishes or (b) the opening of the shell in certain gastropods when the body is retracted | - |
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| operetta | (noun) a short amusing opera | Synonyms: light opera |
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| operon | (noun) a segment of DNA containing adjacent genes including structural genes and an operator gene and a regulatory gene | - |
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| operoseness | (noun) the quality of requiring extended effort | Synonyms: laboriousness, toilsomeness |
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| ophidian | (noun) limbless scaly elongate reptile; some are venomous | Synonyms: serpent, snake |
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| ophidism | (noun) poisoning by snake venom | - |
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| ophiolatry | (noun) the worship of snakes | Synonyms: serpent-worship |
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| ophryon | (noun) the craniometric point in the midline of the forehead immediately above the orbits | - |
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| ophthalmectomy | (noun) surgical removal of an eye | - |
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| ophthalmia | (noun) severe conjunctivitis | Synonyms: ophthalmitis |
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| ophthalmitis | (noun) severe conjunctivitis | Synonyms: ophthalmia |
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| ophthalmologist | (noun) a medical doctor specializing in the diagnosis and treatment of diseases of the eye | Synonyms: eye doctor, oculist |
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| ophthalmology | (noun) the branch of medicine concerned with the eye and its diseases | - |
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| ophthalmoplegia | (noun) paralysis of the motor nerves of the eye | - |
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| ophthalmoscope | (noun) medical instrument for examining the retina of the eye | - |
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| ophthalmoscopy | (noun) examination of the interior of an eye using an ophthalmoscope | - |
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| opiate | (noun) a narcotic drug that contains opium or an opium derivative | - |
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| opinion | (noun) the reason for a court's judgment (as opposed to the decision itself) | Synonyms: ruling |
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| (noun) a personal belief or judgment that is not founded on proof or certainty | Synonyms: persuasion, sentiment, thought, view |
| (noun) a belief or sentiment shared by most people; the voice of the people | Synonyms: popular opinion, public opinion, vox populi |
| (noun) a vague idea in which some confidence is placed | Synonyms: belief, feeling, impression, notion |
| (noun) the legal document stating the reasons for a judicial decision | Synonyms: judgement, judgment, legal opinion |
| (noun) a message expressing a belief about something; the expression of a belief that is held with confidence but not substantiated by positive knowledge or proof | Synonyms: view |
| opisthorchiasis | (noun) infestation with flukes obtained from eating raw fish; common in eastern Asia | - |
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| opisthotonos | (noun) severe spasm in which the back arches and the head bends back and heels flex toward the back | - |
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| opium | (noun) an addictive narcotic extracted from seed capsules of the opium poppy | - |
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| opopanax | (noun) an odorous gum resin formerly used in medicines | - |
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| opossum | (noun) nocturnal arboreal marsupial having a naked prehensile tail found from southern North America to northern South America | Synonyms: possum |
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| (noun) small furry Australian arboreal marsupials having long usually prehensile tails | Synonyms: phalanger, possum |
| oppa | (noun) a male who is older than the speaker, especially used by Korean female | - |
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| opponent | (noun) someone who offers opposition | Synonyms: adversary, antagonist, opposer, resister |
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| (noun) a contestant that you are matched against | Synonyms: opposite, opposition |
| opportuneness | (noun) timely convenience | Synonyms: patness, timeliness |
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| opportunism | (noun) taking advantage of opportunities without regard for the consequences for others | Synonyms: expedience, self-interest, self-seeking |
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| opportunist | (noun) a person who places expediency above principle | Synonyms: self-seeker |
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| opportunity | (noun) a possibility due to a favorable combination of circumstances | Synonyms: chance |
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| opposer | (noun) someone who offers opposition | Synonyms: adversary, antagonist, opponent, resister |
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| opposite | (noun) a word that expresses a meaning opposed to the meaning of another word, in which case the two words are antonyms of each other | Synonyms: antonym, opposite word |
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| (noun) a contestant that you are matched against | Synonyms: opponent, opposition |
| (noun) a relation of direct opposition | Synonyms: contrary, reverse |
| (noun) something inverted in sequence or character or effect | Synonyms: inverse |
| oppositeness | (noun) the relation between opposed entities | Synonyms: opposition |
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| opposition | (noun) the act of hostile groups opposing each other | Synonyms: confrontation |
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| (noun) the action of opposing something that you disapprove or disagree with | Synonyms: resistance |
| (noun) a body of people united in opposing something | - |
| (noun) an armed adversary (especially a member of an opposing military force) | Synonyms: enemy, foe, foeman |
| (noun) a contestant that you are matched against | Synonyms: opponent, opposite |
| (noun) a direction opposite to another | - |
| (noun) the relation between opposed entities | Synonyms: oppositeness |
| oppositionality | (noun) The quality of opposing or of being in conflict. | - |
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| oppression | (noun) the act of subjugating by cruelty | Synonyms: subjugation |
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| (noun) a feeling of being oppressed | Synonyms: oppressiveness |
| (noun) the state of being kept down by unjust use of force or authority: | - |
| oppressiveness | (noun) unwelcome burdensome difficulty | Synonyms: burdensomeness, heaviness, onerousness |
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| (noun) a feeling of being oppressed | Synonyms: oppression |
| oppressor | (noun) a person of authority who subjects others to undue pressures | - |
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| opprobrium | (noun) a state of extreme dishonor | Synonyms: infamy |
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| (noun) state of disgrace resulting from public abuse | Synonyms: obloquy |
| opsin | (noun) retinal protein formed by the action of light on rhodopsin | - |
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| opsonin | (noun) an antibody in blood serum that attaches to invading microorganisms and other antigens to make them more susceptible to the action of phagocytes | - |
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| opsonisation | (noun) process whereby opsonins make an invading microorganism more susceptible to phagocytosis | Synonyms: opsonization |
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| opsonization | (noun) process whereby opsonins make an invading microorganism more susceptible to phagocytosis | Synonyms: opsonisation |
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| optative | (noun) a mood (as in Greek or Sanskrit) that expresses a wish or hope; expressed in English by modal verbs | Synonyms: optative mood |
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| optic | (noun) the organ of sight | Synonyms: eye, oculus |
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| optician | (noun) a worker who makes glasses for remedying defects of vision | Synonyms: lens maker |
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| optics | (noun) optical properties | - |
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| (noun) the branch of physics that studies the physical properties of light | - |
| optimisation | (noun) the act of rendering optimal | Synonyms: optimization |
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| optimism | (noun) a general disposition to expect the best in all things | - |
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| (noun) the optimistic feeling that all is going to turn out well | - |
| optimist | (noun) a person disposed to take a favorable view of things | - |
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| optimization | (noun) the act of rendering optimal | Synonyms: optimisation |
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| optimum | (noun) most favorable conditions or greatest degree or amount possible under given circumstances | - |
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| option | (noun) the act of choosing or selecting | Synonyms: choice, pick, selection |
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| (noun) one of a number of things from which only one can be chosen | Synonyms: alternative, choice |
| (noun) the right to buy or sell property at an agreed price; the right is purchased and if it is not exercised by a stated date the money is forfeited | - |
| optionality | (noun) Quality or state in which choice or discretion is allowed. | Synonyms: electiveness |
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| optometrist | (noun) a person skilled in testing for defects of vision in order to prescribe corrective glasses | Synonyms: oculist |
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| optometry | (noun) the practice of an optometrist | - |
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| opulence | (noun) wealth as evidenced by sumptuous living | Synonyms: luxuriousness, luxury, sumptuousness |
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| opus | (noun) a musical work that has been created | Synonyms: composition, musical composition, piece, piece of music |
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