The Collaborative International Dictionary of English v.0.48:

Idiom \Id"i*om\ ([i^]d"[i^]*[u^]m), n. [F. idiome, L. idioma,
   fr. Gr. 'idi`wma, fr. 'idioy^n to make a person's own, to
   make proper or peculiar; fr. 'i`dios one's own, proper,
   peculiar; prob. akin to the reflexive pronoun o"y^, o'i^,
   'e`, and to "eo`s, 'o`s, one's own, L. suus, and to E. so.]
   1. The syntactical or structural form peculiar to any
      language; the genius or cast of a language.
      [1913 Webster]

            Idiom may be employed loosely and figuratively as a
            synonym of language or dialect, but in its proper
            sense it signifies the totality of the general rules
            of construction which characterize the syntax of a
            particular language and distinguish it from other
            tongues.                              --G. P. Marsh.
      [1913 Webster]

            By idiom is meant the use of words which is peculiar
            to a particular language.             --J. H.
                                                  Newman.
      [1913 Webster]

            He followed their language [the Latin], but did not
            comply with the idiom of ours.        --Dryden.
      [1913 Webster]

   2. An expression conforming or appropriate to the peculiar
      structural form of a language.
      [1913 Webster]

            Some that with care true eloquence shall teach,
            And to just idioms fix our doubtful speech. --Prior.
      [1913 Webster]

   3. A combination of words having a meaning peculiar to itself
      and not predictable as a combination of the meanings of
      the individual words, but sanctioned by usage; as, an
      idiomatic expression; less commonly, a single word used in
      a peculiar sense.
      [1913 Webster +PJC]

            It is not by means of rules that such idioms as the
            following are made current: "I can make nothing of
            it." "He treats his subject home." --Dryden. "It is
            that within us that makes for righteousness." --M.
            Arnold.                               --Gostwick
                                                  (Eng. Gram.)
      [1913 Webster]

            Sometimes we identify the words with the object --
            though by courtesy of idiom rather than in strict
            propriety of language.                --Coleridge.
      [1913 Webster]

   4. The phrase forms peculiar to a particular author; as,
      written in his own idiom.
      [1913 Webster]

            Every good writer has much idiom.     --Landor.
      [1913 Webster]

   5. Dialect; a variant form of a language.
      [1913 Webster]

   Syn: Dialect.

   Usage: Idiom, Dialect. The idioms of a language belong to
          its very structure; its dialects are varieties of
          expression ingrafted upon it in different localities
          or by different professions. Each county of England
          has some peculiarities of dialect, and so have most of
          the professions, while the great idioms of the
          language are everywhere the same. See Language.
          Idiomatic

WordNet (r) 3.0 (2006):

idiom
    n 1: a manner of speaking that is natural to native speakers of
         a language [syn: parlance, idiom]
    2: 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]
    3: the style of a particular artist or school or movement; "an
       imaginative orchestral idiom" [syn: artistic style,
       idiom]
    4: an expression whose meanings cannot be inferred from the
       meanings of the words that make it up [syn: idiom,
       idiomatic expression, phrasal idiom, set phrase,
       phrase]

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "idiom":
   Acadian, Anglo-Indian, Brooklynese, Cajun, Canadian French,
   Cockney, French Canadian, Gullah, Midland, Midland dialect,
   New England dialect, Pennsylvania Dutch, Yankee, Yorkshire,
   adjectival phrase, argot, bundle of isoglosses, cant,
   choice of words, class dialect, clause, cliche, composition,
   construction, dialect, dialect dictionary, diction, expression,
   formulation, grammar, headed group, idiotism, isogloss, jargon,
   language, langue, lingo, lingua, linguistic atlas,
   linguistic community, linguistic island, local dialect, localism,
   locution, manner of speaking, noun phrase, paragraph, parlance,
   parole, patois, peculiar expression, period, personal usage,
   phrasal idiom, phrase, phraseology, phrasing, provincialism,
   regional accent, regionalism, rhetoric, sentence, set phrase,
   speech, speech community, standard phrase, subdialect,
   syntactic structure, talk, term, tongue, turn of expression,
   turn of phrase, usage, use of words, usus loquendi, utterance,
   verb complex, verb phrase, verbalism, verbiage, vernacular,
   way of speaking, word-group, wordage, wording