idiom

a manner of speaking that is natural to native speakers of a language

the usage or vocabulary that is characteristic of a specific group of people; "the immigrants spoke an odd dialect of English"; "he has a strong German accent"; "it has been said that a language is a dialect with an army and navy"

the style of a particular artist or school or movement; "an imaginative orchestral idiom"

an expression whose meanings cannot be inferred from the meanings of the words that make it up