Idiom

1. 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.
2. Dialect; a variant form of a language.
3. The syntactical or structural form peculiar to any language; the genius or cast of a language.