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

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "abbreviation":
   abbreviature, abrege, abridgment, abstract, apocope, aposiopesis,
   astriction, astringency, blue-penciling, bottleneck,
   bowdlerization, brief, cancellation, capsule, censoring,
   censorship, cervix, circumscription, clipping, coarctation,
   compactedness, compaction, compend, compression, compressure,
   concentration, condensation, condensed version, consolidation,
   conspectus, constriction, constringency, contraction, contracture,
   crasis, curtailment, cutting, decrease, deletion, digest,
   diminuendo, draft, editing, elision, ellipsis, epitome, erasure,
   expurgation, foreshortening, head, hourglass, hourglass figure,
   isthmus, knitting, narrow place, narrowing, neck, omission,
   outline, overview, pandect, precis, pruning, puckering, pursing,
   recap, recapitulation, reduction, retrenchment, review, rubric,
   shortened version, shortening, skeleton, sketch, solidification,
   stranglement, strangulation, striction, stricture, striking,
   summary, summation, survey, syllabus, syncope, syneresis, synopsis,
   systole, telescoping, thumbnail sketch, topical outline,
   truncation, wasp waist, wrinkling



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ABBREVIATION, practice.  The omission of some words or letters in writing;
as when fieri facias is written fi. fa.
     2. In writing contracts it is the better practice to make no
abbreviations; but in recognizances, and many other contracts, they are
used; as John Doe tent to prosecute, &c. Richard Roe tent to appear, &c.
when the recognizances are used, they are drawn out in extenso. See 4 Ca. &
P. 61; S.C.19E.C.L.R.268; 9 Co.48.