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.
      [1913 Webster]

   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).
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Define \De*fine"\, v. i.
   To determine; to decide. [Obs.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

define
    v 1: determine the essential quality of [syn: specify,
         define, delineate, delimit, delimitate]
    2: give a definition for the meaning of a word; "Define
       `sadness'"
    3: determine the nature of; "What defines a good wine?"
    4: show the form or outline of; "The tree was clearly defined by
       the light"; "The camera could define the smallest object"
       [syn: define, delineate]
    5: decide upon or fix definitely; "fix the variables"; "specify
       the parameters" [syn: specify, set, determine,
       define, fix, limit]

Moby Thesaurus II by Grady Ward, 1.0:

159 Moby Thesaurus words for "define":
   assign, baptize, be characteristic, bed, blaze, blaze a trail,
   blemish, blotch, bound, brand, call, chalk, chalk up, characterize,
   check, check off, christen, cicatrize, circle in, circumscribe,
   clarify, confirm, construe, dapple, dash, deep-dye, delimit,
   delimitate, delineate, demarcate, denominate, describe, designate,
   detail, determine, diagnose, differentiate, discolor, distinguish,
   dot, dub, earmark, edge, embed, enclose, engraft, engrave, entitle,
   entrench, establish, etch, explain, expound, fix, fleck, found,
   freckle, gash, ground, hatch, hem, identify, impact, implant,
   impress, imprint, infix, ingrain, inscribe, interpret, jam,
   keynote, label, lay down, lay off, limit, line, lodge, make a mark,
   margin, mark, mark boundaries, mark off, mark out,
   mark the periphery, mottle, name, nick, nickname, nominate, notch,
   outline, pack, pencil, pepper, plant, point, prick, print, punch,
   punctuate, puncture, read, read into, riddle, rim, root, rope off,
   scar, scarify, score, scotch, scratch, seal, seam, seat, set,
   set apart, set down, set in, set the limit, set the pace,
   set the tone, settle, skirt, sound the keynote, specify, speck,
   speckle, spell out, splotch, spot, stain, stake out, stamp, state,
   stereotype, stigmatize, streak, striate, stripe, style, surround,
   tag, take, take it that, take to mean, tattoo, term, tick,
   tick off, title, trace, underline, underscore, understand,
   understand by, verge, wedge