From The Collaborative International Dictionary of English v.0.48:

Operation \Op`er*a"tion\, n. [L. operatio: cf. F. op['e]ration.]
   1. The act or process of operating; agency; the exertion of
      power, physical, mechanical, or moral.
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            The pain and sickness caused by manna are the
            effects of its operation on the stomach. --Locke.
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            Speculative painting, without the assistance of
            manual operation, can never attain to perfection.
                                                  --Dryden.
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   2. The method of working; mode of action.
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   3. That which is operated or accomplished; an effect brought
      about in accordance with a definite plan; as, military or
      naval operations.
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   4. Effect produced; influence. [Obs.]
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            The bards . . . had great operation on the vulgar.
                                                  --Fuller.
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   5. (Math.) Something to be done; some transformation to be
      made upon quantities or mathematical objects, the
      transformation being indicated either by rules or symbols.
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   6. (Surg.) Any methodical action of the hand, or of the hand
      with instruments, on the human body, to produce a curative
      or remedial effect, as in amputation, etc.
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   Calculus of operations. See under Calculus.
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From WordNet (r) 2.0:

operation
     n 1: a business especially one run on a large scale; "a
          large-scale farming operation"; "a multinational
          operation"; "they paid taxes on every stage of the
          operation"; "they had to consolidate their operations"
     2: a planned activity involving many people performing various
        actions; "they organized a rescue operation"; "the biggest
        police operation in French history"; "running a restaurant
        is quite an operation"; "consolidate the companies various
        operations"
     3: a process or series of acts especially of a practical or
        mechanical nature involved in a particular form of work;
        "the operations in building a house"; "certain machine
        tool operations" [syn: procedure]
     4: the state of being in effect or being operative; "that rule
        is no longer in operation"
     5: a medical procedure involving an incision with instruments;
        performed to repair damage or arrest disease in a living
        body; "they will schedule the operation as soon as an
        operating room is available"; "he died while undergoing
        surgery" [syn: surgery, surgical operation, surgical
        procedure, surgical process]
     6: activity by a military or naval force (as a maneuver or
        campaign); "it was a joint operation of the navy and air
        force" [syn: military operation]
     7: (computer science) data processing in which the result is
        completely specified by a rule (especially the processing
        that results from a single instruction); "it can perform
        millions of operations per second"
     8: process or manner of functioning or operating; "the power of
        its engine determine its operation"; "the plane's
        operation in high winds"; "they compared the cooking
        performance of each oven"; "the jet's performance
        conformed to high standards" [syn: functioning, performance]
     9: (mathematics) calculation by mathematical methods; "the
        problems at the end of the chapter demonstrated the
        mathematical processes involved in the derivation"; "they
        were learning the basic operations of arithmetic" [syn: mathematical
        process, mathematical operation]
     10: (psychology) the performance of some composite cognitive
         activity; an operation that affects mental contents; "the
         process of thinking"; "the cognitive operation of
         remembering" [syn: process, cognitive process, mental
         process, cognitive operation]
     11: the activity of operating something (a machine or business
         etc.); "her smooth operation of the vehicle gave us a
         surprisingly comfortable ride"

From Moby Thesaurus II by Grady Ward, 1.0:

162 Moby Thesaurus words for "operation":
   accomplished fact, accomplishment, achievement, act, acta, acting,
   action, activism, activity, adventure, affair,
   amphibious operations, amputation, anastomotic operation,
   appliance, application, attempt, behavior, bloodless operation,
   blow, bringing to fruition, business, business deal, campaign,
   capital operation, combined operations, commercial transaction,
   commitment, compensating operation, contract, control,
   corneal transplant, coup, crescent operation, deal, dealings, deed,
   direction, doing, doings, dry run, duty, effective, effectuation,
   efficacious, effort, elective operation, emergency operation,
   employment, end use, endeavor, engagement, enterprise, excision,
   execution, exercise, exercising, exertion, exploit, exploitation,
   exploratory operation, fait accompli, feat, fenestration operation,
   fluid operations, function, functional, functioning, gest, go,
   hand, handiwork, handling, heart transplant, immediate purpose,
   in effect, in force, interval operation, job, kidney transplant,
   logistics, major operation, management, managing, maneuver,
   maneuvers, manipulation, measure, military operations,
   minor operation, minor operations, mission, motion, move, movement,
   movements, negotiation, obligation, occupation, office, operating,
   operational, operational purpose, operations, operative,
   organ transplant, organ transplantation, overproduction,
   overseas operations, overt act, package deal, palliative operation,
   passage, performance, plan, play, practice, praxis, procedure,
   proceeding, production, productiveness, program, project,
   proposition, purpose, radical operation, realization, removal,
   res gestae, resection, role, running, section, staff work, step,
   stroke, stunt, surgical intervention, surgical operation,
   surgical technique, swing, task, the knife, thing, thing done,
   tour de force, transaction, transplant, turn, ultimate purpose,
   undertaking, use, using, utilization, venture, war game, war plans,
   work, working, workings, works