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.
Idiom:
neither fish nor fowl
Meaning:
something that does not belong to a definite group or category
Example:
The man's opinions were neither fish nor fowl and nobody could put them into an identifiable category.
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.