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.
Idiom:
tie up (traffic)
Meaning:
to slow down traffic, to cause road traffic to stop
Example:
The accident tied up traffic for two hours last night.
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.