A pronoun (as `that' or `which' or `who') that introduces a relative clause referring to some antecedent WordNet 3.0 & Open English WordNet

A relative pronoun is a pronoun that marks a relative clause. Wikipedia

Type of pronoun that marks a relative clause Wikidata

A word that introduces a subordinate relative clause within a larger sentence and refers to a previously used noun. OmegaWiki

A pronoun that introduces a relative clause and refers to an antecedent. Some words that can be used as interrogative pronouns can alternatively be used as relative pronouns: what, which, who, and whom. The other relative pronouns are whoever, whomever, whatever, and that. Wiktionary

Pronoun that introduces a relative clause. Wiktionary (translation)