/ ˈɪdiəm; NAmE / noun
1.
[ C ] a group of words whose meaning is different from the meanings of the individual words :
'Let the cat out of the bag' is an idiom meaning to tell a secret by mistake.
➡ note at word
2.
[ U , C ] ( formal ) the kind of language and grammar used by particular people at a particular time or place
3.
[ U , C ] ( formal ) the style of writing, music, art, etc. that is typical of a particular person, group, period or place :
the classical / contemporary / popular idiom
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WORD ORIGIN
late 16th cent.: from French idiome , or via late Latin from Greek idiōma private property, peculiar phraseology, from idiousthai make one's own, from idios own, private.