| 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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