An idiom is a group of words, or in other words, a phrase that has a meaning other than its literal meaning of the words in it.
Idiom:
stop off (somewhere)
Meaning:
to stop at a place for a short time while going somewhere
Example:
We decided to stop off at the fish store before we went home.
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.