Answer:
An Idiom.
Step-by-step explanation:
An idiom can be defined as a group of words whose meaning can not be inferred from its literal meaning. It is an expression and the meaning of this expression can be understood implicitly.
For example, kill two birds with one stone is an idiom whose literal meaning can be inferred of actual killing of two birds with one stone. But the true meaning of this expression is to perform an action that will fulfill two responsibilities at the same time without extra effort.
So, the correct answer is an idiom.