The Collaborative International Dictionary of English v.0.48:

Alter \Al"ter\, v. t. [imp. & p. p. Altered; p. pr. & vb. n.
   Altering.] [F. alt['e]rer, LL. alterare, fr. L. alter
   other, alius other. Cf. Else, Other.]
   1. To make otherwise; to change in some respect, either
      partially or wholly; to vary; to modify. "To alter the
      king's course." "To alter the condition of a man." "No
      power in Venice can alter a decree." --Shak.
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            It gilds all objects, but it alters none. --Pope.
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            My covenant will I not break, nor alter the thing
            that is gone out of my lips.          --Ps. lxxxix.
                                                  34.
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   2. To agitate; to affect mentally. [Obs.] --Milton.
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   3. To geld. [Colloq.]
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   Syn: Change, Alter.

   Usage: Change is generic and the stronger term. It may
          express a loss of identity, or the substitution of one
          thing in place of another; alter commonly expresses a
          partial change, or a change in form or details without
          destroying identity.
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WordNet (r) 3.0 (2006):

altering
    n 1: the sterilization of an animal; "they took him to the vet
         for neutering" [syn: neutering, fixing, altering]