Idiom
Etymology
From Greek idiōma, "a peculiarity, a peculiar phrasing," from idios ("one’s own, private, peculiar"). The same root gives us "idiosyncrasy" and, distantly, "idiot" (originally a private citizen). The thread is ownership: an idiom is a turn of phrase that belongs peculiarly to one language and doesn’t travel word for word.
noun · /ID-ee-um/
A fixed expression whose meaning can’t be worked out from the individual words in it.
What it names
The phrases that quietly defeat literal translation — "kick the bucket," "under the weather" — and mark you as an insider to a language when you use them naturally.
Example
Learners can master the grammar and still be floored by an idiom like "it’s not rocket science."
Related idea
A cliché is an idiom or phrase worn dull by overuse; an idiom is simply non-literal, and can be vivid or tired. "Raining cats and dogs" is both.
noun · /ID-ee-um/
The characteristic style or mode of expression of a group, period, or art form.
What it names
a distinctive artistic or linguistic manner, as in “the jazz idiom”
Example
The building is designed in a spare modern idiom.
Related idea
Like a group’s vernacular, but applied to style rather than to everyday speech.