What is the definition of relative pronouns?

A relative pronoun introduces a relative clause, which relates its antecedent, giving more information about that antecedent.

The relative pronouns are which, that, who, whom, and whose.

Who and whom refer only to people.

Which refers to things, qualities, and ideas (never to people).

That and whose refer to people, things, qualities, and ideas.

Examples:

Mr. Green, who is my neighbor, gave me some carrots from his garden.

The book that I borrowed from the library is overdue.

I sold my radio, which was very old.

Louisa is the player who pitched first.

The people to whom we sent invitations are all current customers.