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.
skip out on (someone or something)
to sneak away from someone or some event
I decided to skip out on the meeting and go to a movie.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is defined as - a group of words whose meaning is different from the meanings of the individual words.