The Collaborative International Dictionary of English v.0.48:

Substantiate \Sub*stan"ti*ate\, v. t. [imp. & p. p.
   Substantiated; p. pr. & vb. n. Substantiating.]
   1. To make to exist; to make real. --Ayliffe.
      [1913 Webster]

   2. To establish the existence or truth of by proof or
      competent evidence; to verify; as, to substantiate a
      charge or allegation; to substantiate a declaration.
      [1913 Webster]

            Observation is, in turn, wanted to direct and
            substantiate the course of experiment. --Coleridge.
      [1913 Webster]

WordNet (r) 3.0 (2006):

substantiate
    v 1: establish or strengthen as with new evidence or facts; "his
         story confirmed my doubts"; "The evidence supports the
         defendant" [syn: confirm, corroborate, sustain,
         substantiate, support, affirm] [ant: contradict,
         negate]
    2: represent in bodily form; "He embodies all that is evil wrong
       with the system"; "The painting substantiates the feelings of
       the artist" [syn: incarnate, body forth, embody,
       substantiate]
    3: make real or concrete; give reality or substance to; "our
       ideas must be substantiated into actions" [syn: realize,
       realise, actualize, actualise, substantiate]
    4: solidify, firm, or strengthen; "The president's trip will
       substantiate good relations with the former enemy country"

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "substantiate":
   adduce, affirm, anatomize, assay, atomize, attest, authenticate,
   back, back up, bear out, body, bolster, bring to test, buttress,
   certify, circumstantiate, cite, confirm, corporealize, corporify,
   corroborate, cut and try, demonstrate, descend to particulars,
   detail, document, embody, enter into detail, essay, experiment,
   exteriorize, externalize, fortify, give a try, give a tryout,
   give full particulars, have a go, incarnate, incorporate, instance,
   itemize, justify, manifest, materialize, objectify, particularize,
   personalize, personify, play around with, practice upon, probate,
   prove, put to trial, ratify, reembody, reincarnate, reinforce,
   research, road-test, run a sample, sample, shake down, specify,
   spell out, strengthen, substantialize, substantify, support,
   sustain, taste, test, transmigrate, try, try it on, try out,
   undergird, uphold, validate, verify, warrant