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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. [1913 Webster] The pain and sickness caused by manna are the effects of its operation on the stomach. --Locke. [1913 Webster] Speculative painting, without the assistance of manual operation, can never attain to perfection. --Dryden. [1913 Webster] 2. The method of working; mode of action. [1913 Webster] 3. That which is operated or accomplished; an effect brought about in accordance with a definite plan; as, military or naval operations. [1913 Webster] 4. Effect produced; influence. [Obs.] [1913 Webster] The bards . . . had great operation on the vulgar. --Fuller. [1913 Webster] 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. [1913 Webster] 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. [1913 Webster] Calculus of operations. See under Calculus. [1913 Webster]