Idiom

n.
  1. The syntactical or structural form peculiar to any language; the genius or cast of a language.
  2. An expression conforming or appropriate to the peculiar structural form of a language.
  3. A combination of words having a meaning peculiar to itself and not predictable as a combination of the meanings of the individual words, but sanctioned by usage; as, an idiomatic expression; less commonly, a single word used in a peculiar sense.
  4. The phrase forms peculiar to a particular author; as, written in his own idiom.
  5. Dialect; a variant form of a language.