The Collaborative International Dictionary of English v.0.48:

Define \De*fine"\, v. t. [imp. & p. p. Defined; p. pr. & vb.
   n. Defining.] [OE. definer, usually, to end, to finish, F.
   d['e]finir to define, L. definire to limit, define; de- +
   finire to limit, end, finis boundary, limit, end. See
   Final, Finish.]
   1. To fix the bounds of; to bring to a termination; to end.
      "To define controversies." --Barrow.
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   2. To determine or clearly exhibit the boundaries of; to mark
      the limits of; as, to define the extent of a kingdom or
      country.
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   3. To determine with precision; to mark out with
      distinctness; to ascertain or exhibit clearly; as, the
      defining power of an optical instrument.
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            Rings . . . very distinct and well defined. --Sir I.
                                                  Newton.
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   4. To determine the precise signification of; to fix the
      meaning of; to describe accurately; to explain; to expound
      or interpret; as, to define a word, a phrase, or a
      scientific term.
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            They define virtue to be life ordered according to
            nature.                               --Robynson
                                                  (More's
                                                  Utopia).
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WordNet (r) 3.0 (2006):

defining
    n 1: any process serving to define the shape of something [syn:
         shaping, defining]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "defining":
   categorical, characteristic, classificational, classificatory,
   confining, definitive, denominative, differential, distinctive,
   divisional, divisionary, exclusive, limitary, limitative, limiting,
   ordinal, particular, peculiar, restricting, restrictive, special,
   specific, subdivisional, taxonomic, typal, typical