| 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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| orach | (noun) any of various herbaceous plants of the genus Atriplex that thrive in deserts and salt marshes | Synonyms: orache |
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| orache | (noun) any of various herbaceous plants of the genus Atriplex that thrive in deserts and salt marshes | Synonyms: orach |
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| oracle | (noun) a shrine where an oracular god is consulted | - |
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| (noun) a prophecy (usually obscure or allegorical) revealed by a priest or priestess; believed to be infallible | - |
| (noun) an authoritative person who divines the future | Synonyms: prophesier, prophet, seer, vaticinator |
| oral | (noun) an examination conducted by spoken communication | Synonyms: oral exam, oral examination, viva, viva voce |
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| orang | (noun) large long-armed ape of Borneo and Sumatra having arboreal habits | Synonyms: orangutan, orangutang, Pongo pygmaeus |
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| orange | (noun) orange color or pigment; any of a range of colors between red and yellow | Synonyms: orangeness |
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| (noun) round yellow to orange fruit of any of several citrus trees | - |
| (noun) any citrus tree bearing oranges | Synonyms: orange tree |
| (noun) any pigment producing the orange color | - |
| orangeade | (noun) sweetened beverage of diluted orange juice | - |
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| orangeness | (noun) orange color or pigment; any of a range of colors between red and yellow | Synonyms: orange |
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| orangery | (noun) a place where oranges are grown; a plantation of orange trees in warm climes or a greenhouse in cooler areas | - |
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| orangewood | (noun) fine-grained wood of an orange tree; used in fine woodwork | - |
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| orangutan | (noun) large long-armed ape of Borneo and Sumatra having arboreal habits | Synonyms: orang, orangutang, Pongo pygmaeus |
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| orangutang | (noun) large long-armed ape of Borneo and Sumatra having arboreal habits | Synonyms: orang, orangutan, Pongo pygmaeus |
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| oration | (noun) an instance of oratory | - |
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| orator | (noun) a person who delivers a speech or oration | Synonyms: public speaker, rhetorician, speechifier, speechmaker |
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| oratorio | (noun) a musical composition for voices and orchestra based on a religious text | Synonyms: cantata |
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| oratory | (noun) addressing an audience formally (usually a long and rhetorical address and often pompous) | - |
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| orb | (noun) the ball-shaped capsule containing the vertebrate eye | Synonyms: eyeball |
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| (noun) an object with a spherical shape | Synonyms: ball, globe |
| orbit | (noun) an area in which something acts or operates or has power or control: | Synonyms: ambit, compass, range, reach, scope |
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| (noun) the bony cavity in the skull containing the eyeball | Synonyms: cranial orbit, eye socket, orbital cavity |
| (noun) the (usually elliptical) path described by one celestial body in its revolution about another | Synonyms: celestial orbit |
| (noun) the path of an electron around the nucleus of an atom | Synonyms: electron orbit |
| (noun) a particular environment or walk of life | Synonyms: area, arena, domain, field, sphere |
| orbitale | (noun) the craniometric point at the lowest point on the lower edge of the orbit | Synonyms: orbital point |
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| orbiter | (noun) man-made equipment that orbits around the earth or the moon | Synonyms: artificial satellite, satellite |
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| orca | (noun) predatory black-and-white toothed whale with large dorsal fin; common in cold seas | Synonyms: grampus, killer, killer whale, Orcinus orca, sea wolf |
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| orchard | (noun) garden consisting of a small cultivated wood without undergrowth | Synonyms: grove, plantation, woodlet |
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| orchestra | (noun) seating on the main floor in a theater | - |
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| (noun) a musical organization consisting of a group of instrumentalists including string players | - |
| orchestration | (noun) the act of arranging a piece of music for an orchestra and assigning parts to the different musical instruments | Synonyms: instrumentation |
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| (noun) an arrangement of events that attempts to achieve a maximum effect | - |
| (noun) an arrangement of a piece of music for performance by an orchestra or band | - |
| orchestrator | (noun) an arranger who writes for orchestras | - |
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| orchid | (noun) any of numerous plants of the orchid family usually having flowers of unusual shapes and beautiful colors | Synonyms: orchidaceous plant |
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| orchidalgia | (noun) pain in the testes | - |
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| orchidectomy | (noun) surgical removal of one or both testicles | Synonyms: orchiectomy |
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| orchiectomy | (noun) surgical removal of one or both testicles | Synonyms: orchidectomy |
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| orchil | (noun) any of various lecanoras that yield the dye archil | Synonyms: archil |
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| (noun) a purplish dye obtained from orchil lichens | Synonyms: archil, cudbear |
| orchiopexy | (noun) operation to bring an undescended testicle into the scrotum | - |
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| orchis | (noun) one of the two male reproductive glands that produce spermatozoa and secrete androgens | Synonyms: ball, ballock, bollock, egg, nut, testicle, testis |
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| (noun) any of various deciduous terrestrial orchids having fleshy tubers and flowers in erect terminal racemes | - |
| orchitis | (noun) inflammation of one or both testes; characterized by pain and swelling | - |
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| orchotomy | (noun) surgical incision into the testis to obtain material for analysis (as in cases of abnormally low sperm count) | - |
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| ordainer | (noun) a cleric who ordains; a cleric who admits someone to holy orders | - |
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| ordeal | (noun) a primitive method of determining a person's guilt or innocence by subjecting the accused person to dangerous or painful tests believed to be under divine control; escape was usually taken as a sign of innocence | Synonyms: trial by ordeal |
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| (noun) a severe or trying experience | - |
| order | (noun) the act of putting things in a sequential arrangement | Synonyms: ordering |
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| (noun) (architecture) one of original three styles of Greek architecture distinguished by the type of column and entablature used or a style developed from the original three by the Romans | - |
| (noun) a degree in a continuum of size or quantity | Synonyms: order of magnitude |
| (noun) a legally binding command or decision entered on the court record (as if issued by a court or judge) | Synonyms: decree, edict, fiat, rescript |
| (noun) a commercial document used to request someone to supply something in return for payment and providing specifications and quantities | Synonyms: purchase order |
| (noun) a body of rules followed by an assembly | Synonyms: parliamentary law, parliamentary procedure, rules of order |
| (noun) (often plural) a command given by a superior (e.g., a military or law enforcement officer) that must be obeyed | - |
| (noun) a request for something to be made, supplied, or served | - |
| (noun) logical or comprehensible arrangement of separate elements | Synonyms: ordering, ordination |
| (noun) (biology) taxonomic group containing one or more families | - |
| (noun) a formal association of people with similar interests | Synonyms: club, gild, guild, lodge, social club, society |
| (noun) a group of person living under a religious rule | Synonyms: monastic order |
| (noun) established customary state (especially of society) | - |
| (noun) a condition of regular or proper arrangement | Synonyms: orderliness |
| orderer | (noun) an organizer who puts things in order | Synonyms: systematiser, systematist, systematizer, systemiser, systemizer |
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| (noun) someone who places an order to buy | - |
| ordering | (noun) the act of putting things in a sequential arrangement | Synonyms: order |
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| (noun) logical or comprehensible arrangement of separate elements | Synonyms: order, ordination |
| orderliness | (noun) the quality of appreciating method and system | Synonyms: methodicalness |
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| (noun) a condition of regular or proper arrangement | Synonyms: order |
| orderly | (noun) a male hospital attendant who has general duties that do not involve the medical treatment of patients | Synonyms: hospital attendant |
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| (noun) a soldier who serves as an attendant to a superior officer | - |
| ordinal | (noun) the number designating place in an ordered sequence | Synonyms: no., ordinal number |
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| ordinance | (noun) the act of ordaining; the act of conferring (or receiving) holy orders | Synonyms: ordination |
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| (noun) an authoritative command | Synonyms: regulation |
| (noun) a statute enacted by a city government | - |