Idiom (language structure)

An idiom (the quality of it being known as idiomaticness or idiomaticity) is a syntactical, grammatical, or phonological structure peculiar to a language that is actually realized, as opposed to possible but unrealized structures that could have developed to serve the same semantic functions but did not. The grammar of a language (its morphology, phonology, and syntax) is inherently arbitrary and peculiar to a specific language (or group of related languages).

Source: Wikipedia — Idiom (language structure) (CC BY-SA 4.0)