If one thing complements another, it goes well with the other thing and makes its good qualities more noticeable.

If people or things complement each other, they are different or do something different, which makes them a good combination.

Something that is a complement to something else complements it.

The complement of things or people that something has is the number of things or people that it normally has, which enable it to function properly.

In grammar, the complement of a link verb is an adjective group or noun group which comes after the verb and describes or identifies the subject. For example, in the sentence 'They felt very tired', 'very tired' is the complement. In 'They were students', 'students' is the complement.