Source : Webster's Revised Unabridged Dictionary (1913)

Idiom \Id"i*om\, n. [F. idiome, L. idioma, fr. Gr. ?, fr. ? to
   make a person's own, to make proper or peculiar; prob. akin
   to the reflexive pronoun ?, ?, ?, and to ?, ?, 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.

            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.

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

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

   2. An expression conforming or appropriate to the peculiar
      structural form of a language; in extend use, an
      expression sanctioned by usage, having a sense peculiar to
      itself and not agreeing with the logical sense of its
      structural form; also, the phrase forms peculiar to a
      particular author.

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

            Sometimes we identify the words with the object --
            though be courtesy of idiom rather than in strict
            propriety of language.                --Coleridge.

            Every good writer has much idiom.     --Landor.

            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. )

   3. Dialect; a variant form of a language.

   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}.

Source : WordNet®

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