A definition in language explains what a word or phrase means. Defining means giving a definition.

Other words with this meaning are description and explanation. They describe what a word means and explains to the person when and where it can be used.

In mathematics, a definition is an exact way of saying what a mathematical concept is. It might not be the easiest way to say what it is, but it is used because it is exact. It can be used in a mathematical proof.