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.
keep one's head
to stay calm when there is trouble or danger
The president is a very good leader and is able to keep his head during an emergency.
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.