Idiom n. : The syntactical or structural form peculiar to any language; the genius or cast of a language.

Idiom n. : 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.

Idiom n. : Dialect; a variant form of a language.

Example:
What is the difference between a metaphor and an Idiom?

Taxonomy
idiom, Four-character idiom, metaphors, language, free encyclopedia, Search, profits, English expression, Chinese characters, common use, listener, new language, idiomatic expressions, conceptual metaphors, Articles with unsourced statements, figure of speech, true meanings, battle.

Defination from the web:
idiom is a phrase whose meaning cannot be determined by the literal definition of the phrase itself, but refers instead to a figurative ... That is, the collocated words develop a specialized meaning as a whole and an idiom is born. ...