The Collaborative International Dictionary of English v.0.48:

Abbreviation \Ab*bre`vi*a"tion\, n. [LL. abbreviatio: cf. F.
   abbr['e]viation.]
   1. The act of shortening, or reducing.
      [1913 Webster]

   2. The result of abbreviating; an abridgment. --Tylor.
      [1913 Webster]

   3. The form to which a word or phrase is reduced by
      contraction and omission; a letter or letters, standing
      for a word or phrase of which they are a part; as, Gen.
      for Genesis; U.S.A. for United States of America.
      [1913 Webster]

   4. (Mus.) One dash, or more, through the stem of a note,
      dividing it respectively into quavers, semiquavers, or
      demi-semiquavers. --Moore.
      [1913 Webster]

WordNet (r) 3.0 (2006):

abbreviation
    n 1: a shortened form of a word or phrase
    2: shortening something by omitting parts of it