The Collaborative International Dictionary of English v.0.48:

Significant \Sig*nif"i*cant\, a. [L. significans, -antis, p. pr.
   of significare. See Signify.]
   1. Fitted or designed to signify or make known something
      having a meaning; standing as a sign or token; expressive
      or suggestive; as, a significant word or sound; a
      significant look.
      [1913 Webster]

            It was well said of Plotinus, that the stars were
            significant, but not efficient.       --Sir W.
                                                  Raleigh.
      [1913 Webster]

   2. Deserving to be considered; important; momentous; as, a
      significant event.
      [1913 Webster]

   Significant figures (Arith.), the figures which remain to
      any number, or decimal fraction, after the ciphers at the
      right or left are canceled. Thus, the significant figures
      of 25,000, or of .0025, are 25.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Significant \Sig*nif"i*cant\, n.
   That which has significance; a sign; a token; a symbol.
   --Wordsworth.
   [1913 Webster]

         In dumb significants proclaim your thoughts. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

significant
    adj 1: important in effect or meaning; "a significant change in
           tax laws"; "a significant change in the Constitution"; "a
           significant contribution"; "significant details";
           "statistically significant" [syn: significant,
           important] [ant: insignificant, unimportant]
    2: fairly large; "won by a substantial margin" [syn:
       significant, substantial]
    3: too closely correlated to be attributed to chance and
       therefore indicating a systematic relation; "the interaction
       effect is significant at the .01 level"; "no significant
       difference was found" [ant: nonsignificant]
    4: rich in significance or implication; "a meaning look" [syn:
       meaning(a), pregnant, significant]

Moby Thesaurus II by Grady Ward, 1.0:

174 Moby Thesaurus words for "significant":
   absolute, adducible, admissible, allegorical, associational,
   attestative, attestive, augural, authentic, based on, big,
   big-league, big-name, big-time, bigwig, bigwigged, certain,
   circumstantial, cogent, compelling, conclusive, connotational,
   connotative, consequential, considerable, conspicuous, convincing,
   critical, cumulative, damning, decisive, definable, demonstrative,
   denominative, denotational, denotative, designative, determinative,
   diagnostic, documentary, documented, double-barreled, earthshaking,
   eloquent, emblematic, evidence, evidential, evidentiary, ex parte,
   exhibitive, expressive, extended, extensional, eye-witness,
   factual, facund, figural, figurative, final, firsthand, forceful,
   forerunning, foreshadowing, foreshowing, foretokening, forewarning,
   founded on, full of meaning, full of point, full of substance,
   grand, great, grounded on, hearsay, heavy with meaning,
   heavyweight, high-powered, historic, identifying, ideographic,
   idiosyncratic, implicative, implicit, important, impressive,
   in the limelight, incontrovertible, index, indicating, indicative,
   indicatory, indisputable, individual, informative, intelligible,
   intensional, interpretable, intuitive, irrefutable, irresistible,
   major, mark, material, meaning, meaningful, meaty, meritorious,
   metaphorical, momentous, monitory, name, naming, notable,
   noteworthy, nuncupative, outstanding, overwhelming, pathognomonic,
   peculiar, pithy, pointed, powerful, precursive, precursory,
   predictive, prefigurative, pregnant, preindicative, premonitory,
   presageful, presaging, presumptive, probative, prognostic,
   prognosticative, prominent, readable, referential, relevant,
   reliable, representative, rich, self-important, semantic, semiotic,
   sententious, sign, signal, signalizing, significative, signifying,
   sound, substantial, substantive, suggestive, superior, sure,
   symbolic, symbolistic, symbological, symptom, symptomatic,
   symptomatologic, telling, to the front, token, transferred,
   typical, valid, valuable, valued, warning, weighty,
   world-shaking