an expression that has become established in a language and that has a meaning that cannot be derived from the meanings of its individual elements, e.g. have bats in one's belfry meaning to be mad or eccentric
the language peculiar to a people or to a district, community, or class
the syntactic, grammatical, or structural form peculiar to a language
a characteristic style or form of artistic expression
the modern jazz idiom
[French idiome via Latin from Greek idiōma individual peculiarity of language, from idios one's own, private]