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.
grain of truth
the smallest amount of truth
There was not a grain of truth in what the man said.
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 other than its literal meaning of the words in it.