A relative clause is a clause that modifies a noun or noun phrase and uses some grammatical device to indicate that one of the arguments in the relative clause refers to the noun or noun phrase. For example, in the sentence I met a man who wasn't too sure of himself, the subordinate clause who wasn't too sure of himself is a relative clause since it modifies the noun man and uses the pronoun who to indicate that the same "man" is referred to in the subordinate clause (in this case as its subject).

In many European languages, relative clauses are introduced by a special class of pronouns called relative pronouns, such as who in the example just given. In other languages, relative clauses may be marked in different ways: they may be introduced by a special class of conjunctions called relativizers, the main verb of the relative clause may appear in a special morphological variant, or a relative clause may be indicated by word order alone. In some languages, more than one of these mechanisms may be possible.