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.
Idiom:
a known quantity
Meaning:
someone or something that is known and understood
Example:
The new employee is a known quantity in our office because he worked here before.
List of common idioms, their meanings and examples of their usage in sentences
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.