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.
Idiom:
see a man about a dog
Meaning:
to leave for some unspecified purpose (often to go to the restroom)
Example:
I drank several cups of coffee and I soon had to stop my car to see a man about a dog.
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.