What is a literal meaning of idiom?

The syntactical or structural form peculiar to any language; the genius or cast of a language., 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., Dialect; a variant form of a language.