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.
Idiom:
Tell it to the marines.
Meaning:
I do not believe you, stop trying to fool me
Example:
My friend said that she was going to start her own business. I told her to tell it to the marines.
List of common idioms, their meanings and examples of their usage in sentences
An expression in the usage of a language that is peculiar to itself either in having a meaning that cannot be derived from the conjoined meanings of its elements or in its grammatically atypical use of words.