n.
  - 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.
 - a characteristic mode of expression in music or art.
 
 
  Derivative
  
    - idiomatic adj.
  - idiomatically adv.
 
   
  Etymology
  C16: from Fr. idiome, or via late L. from Gk idiōma ‘private property’.