id|i|om [ˈıdiəm] n
[Date: 1500-1600; : French; Origin: idiome, from Greek idioma 'personal way of expressing yourself', from idios; IDIOT]
a group of words that has a special meaning that is different from the ordinary meaning of each separate word. For example, 'under the weather' is an idiom meaning 'ill'.
2.) [U and C] formal or technical a style of expression in writing, speech, or music that is typical of a particular group of people
the new musical idiom

Dictionary of contemporary English. 2013.

(sanctioned by use in any language) / (of a language), /