The Collaborative International Dictionary of English v.0.48:

Definition \Def`i*ni"tion\, n. [L. definitio: cf. F.
   d['e]finition.]
   1. The act of defining; determination of the limits; as, a
      telescope accurate in definition.
      [1913 Webster]

   2. Act of ascertaining and explaining the signification; a
      description of a thing by its properties; an explanation
      of the meaning of a word or term; as, the definition of
      "circle;" the definition of "wit;" an exact definition; a
      loose definition.
      [1913 Webster]

            Definition being nothing but making another
            understand by words what the term defined stands
            for.                                  --Locke.
      [1913 Webster]

   3. Description; sort. [R.] "A new creature of another
      definition." --Jer. Taylor.
      [1913 Webster]

   4. (Logic) An exact enunciation of the constituents which
      make up the logical essence.
      [1913 Webster]

   5. (Opt.) Distinctness or clearness, as of an image formed by
      an optical instrument; precision in detail.

   Syn: Definition, Explanation, Description.

   Usage: A definition is designed to settle a thing in its
          compass and extent; an explanation is intended to
          remove some obscurity or misunderstanding, and is
          therefore more extended and minute; a description
          enters into striking particulars with a view to
          interest or impress by graphic effect. It is not
          therefore true, though often said, that description is
          only an extended definition. "Logicians distinguish
          definitions into essential and accidental. An
          essential definition states what are regarded as the
          constituent parts of the essence of that which is to
          be defined; and an accidental definition lays down
          what are regarded as circumstances belonging to it,
          viz., properties or accidents, such as causes,
          effects, etc." --Whately.
          [1913 Webster]

WordNet (r) 3.0 (2006):

definition
    n 1: a concise explanation of the meaning of a word or phrase or
         symbol
    2: clarity of outline; "exercise had given his muscles superior
       definition"

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "definition":
   acutance, appellation, atmospheric visibility, baptism, black spot,
   bloom, blooping, bounding, calling, ceiling, ceiling unlimited,
   characterization, christening, circling-in, circumscribing,
   circumscription, clarification, clarity, clear-cutness, clearness,
   coherence, connectedness, consistency, conspicuity,
   conspicuousness, construction, crystal-clearness, crystallinity,
   definiteness, delimitation, delineation, demarcation, denomination,
   description, designation, determination, diagnosis,
   differentiation, directness, distinction, distinctness, encincture,
   evidence, evidentness, explanation, explication, explicitness,
   flare, focus, fringe area, ghost, granulation, grid, hard shadow,
   high visibility, identification, image, inclusion-exclusion,
   interpretation, limpidity, low visibility, lucidity, manifestness,
   meaning, multiple image, naming, nicknaming, noise, obviousness,
   outlining, patentness, pellucidity, percipi, perspicuity, picture,
   picture noise, picture shifts, plain English, plain speech,
   plain style, plainness, precision, prominence, rain, reading,
   resolution, rolling, scanning pattern, scintillation, seeing,
   sense, shading, sharpness, simplicity, snow, snowstorm,
   specification, straightforwardness, structure, styling, terming,
   transparency, transpicuity, unadorned style, unambiguousness,
   unequivocalness, unmistakableness, visibility unlimited,
   visibility zero, way of seeing