A group of words which have a different meaning when used together from the one they would have if you took the meaning of each word separately.
use one's head/bean/noodle/noggin
to think carefully about something
"You should use your head a little more and try not to make the same mistake again."
List of common idioms, their meanings and examples of their usage in sentences
An idiom is a widely used saying or expression containing a figurative meaning that differs from the phrase's literal meaning.