idiom

Concise Oxford English Dictionary © 2008 Oxford University Press:
idiom/ˈɪdiəm/

▶noun

• 1 a group of words established by usage as having a meaning not deducible from those of the individual words (e.g. over the moon).

  ■ a form of expression natural to a language, person, or group.

• 2 a characteristic mode of expression in music or art.

– derivatives
idiomatic adjective,
idiomatically adverb.

– origin C16: from Fr. idiome, or via late L. from Gk idiōma ‘private property’.