The Collaborative International Dictionary of English v.0.48:

Expatiate \Ex*pa"ti*ate\, v. i. [imp. & p. p. Expatiated; p.
   pr. & vb. n. Expariating.] [L. expatiatus, exspatiatus, p.
   p. of expatiari, exspatiari, to expatiate; ex out + spatiari
   to walk about spread out, fr. spatium space. See Space.]
   1. To range at large, or without restraint.
      [1913 Webster]

            Bids his free soul expatiate in the skies. --Pope.
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   2. To enlarge in discourse or writing; to be copious in
      argument or discussion; to descant.
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            He expatiated on the inconveniences of trade.
                                                  --Addison.
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The Collaborative International Dictionary of English v.0.48:

Expatiate \Ex*pa"ti*ate\, v. t.
   To expand; to spread; to extend; to diffuse; to broaden.
   [1913 Webster]

         Afford art an ample field in which to expatiate itself.
                                                  --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

expatiate
    v 1: add details, as to an account or idea; clarify the meaning
         of and discourse in a learned way, usually in writing; "She
         elaborated on the main ideas in her dissertation" [syn:
         elaborate, lucubrate, expatiate, exposit,
         enlarge, flesh out, expand, expound, dilate]
         [ant: abbreviate, abridge, contract, cut,
         foreshorten, reduce, shorten]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "expatiate":
   amplify, descant, detail, develop, dilate, discuss, dissert,
   dissertate, elaborate, enlarge, enlarge upon, evolve, expand,
   explicate, narrate, particularize, ramble, recite, recount,
   rehearse, rehearse in extenso, relate, relate at large, sermonize,
   unfold, work out