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.
build up to (something)
to lead up to something
Things were building up to be a very serious problem.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is defined as - a group of words in a fixed order that has a particular meaning that is different from the meanings of each word on its own.