The Oxford Thesaurus An A-Z Dictionary of Synonyms - idiom n.

1) language, tongue, speech, vernacular, dialect, argot, patois, jargon, cant, idiolect, parlance, fa‡on de parler, phraseology: The play is written in a rather old-fashioned idiom. 2) expression, (set) phrase, phrasing, locution, clich‚: The term red herring , an idiom meaning 'false trail', is used of something which is neither red nor a herring.