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.
to be an easy situation
It was clear sailing after we finished work and began our holiday.
List of common idioms, their meanings and examples of their usage in sentences
An expression in the usage of a language that is peculiar to itself either in having a meaning that cannot be derived from the conjoined meanings of its elements or in its grammatically atypical use of words.