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.
in the saddle
in command, in control
The president is back in the saddle after being ill for several months.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is a group of words, or in other words, a phrase that has a meaning different from the literal meaning of the words in it.