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"