The Collaborative International Dictionary of English v.0.48:

Dialect \Di"a*lect\, n. [F. dialecte, L. dialectus, fr. Gr. ?,
   fr. ? to converse, discourse. See Dialogue.]
   1. Means or mode of expressing thoughts; language; tongue;
      form of speech.
      [1913 Webster]

            This book is writ in such a dialect
            As may the minds of listless men affect.
            Bunyan.
            The universal dialect of the world.   --South.
      [1913 Webster]

   2. The form of speech of a limited region or people, as
      distinguished from ether forms nearly related to it; a
      variety or subdivision of a language; speech characterized
      by local peculiarities or specific circumstances; as, the
      Ionic and Attic were dialects of Greece; the Yorkshire
      dialect; the dialect of the learned.
      [1913 Webster]

            In the midst of this Babel of dialects there
            suddenly appeared a standard English language.
                                                  --Earle.
      [1913 Webster]

            [Charles V.] could address his subjects from every
            quarter in their native dialect.      --Prescott.

   Syn: Language; idiom; tongue; speech; phraseology. See
        Language, and Idiom.
        [1913 Webster]

WordNet (r) 3.0 (2006):

dialect
    n 1: the usage or vocabulary that is characteristic of a
         specific group of people; "the immigrants spoke an odd
         dialect of English"; "he has a strong German accent"; "it
         has been said that a language is a dialect with an army and
         navy" [syn: dialect, idiom, accent]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "dialect":
   Acadian, Anglo-Indian, Brooklynese, Cajun, Canadian French,
   Cockney, French Canadian, Gullah, Midland, Midland dialect,
   New England dialect, Pennsylvania Dutch, Yankee, Yorkshire, accent,
   argot, brogue, bundle of isoglosses, burr, cant, choice of words,
   class dialect, composition, dialect atlas, dialect dictionary,
   dialectal, diction, expression, formulation, grammar, idiom,
   idiomatic, isogloss, jargon, language, langue, lingo, lingua,
   linguistic atlas, linguistic community, linguistic island, local,
   local dialect, localism, locution, parlance, parole, patois,
   patter, personal usage, phrase, phraseology, phrasing, pidgin,
   pronunciation, provincial, provincialism, regional,
   regional accent, regionalism, rhetoric, slang, speech,
   speech community, subdialect, talk, tongue, usage, use of words,
   usus loquendi, verbiage, vernacular, wordage, wording