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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WordNet (r) 3.0 (2006):

operation
    n 1: the state of being in effect or being operative; "that rule
         is no longer in operation"
    2: 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"
    3: 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"
    4: (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"
    5: activity by a military or naval force (as a maneuver or
       campaign); "it was a joint operation of the navy and air
       force" [syn: operation, military operation]
    6: 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: operation, surgery, surgical operation,
       surgical procedure, surgical process]
    7: 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: operation, procedure]
    8: process or manner of functioning or operating; "the power of
       its engine determines 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: operation, functioning, performance]
    9: (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, operation, cognitive operation]
    10: (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,
        operation]
    11: the activity of operating something (a machine or business
        etc.); "her smooth operation of the vehicle gave us a
        surprisingly comfortable ride"

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