In linguistics, a relativizer is a type of conjunction that introduces a relative clause. For example, in English, the conjunction that may be considered a relativizer in a sentence such as "I have one that you can use." Relativizers do not appear, at least overtly, in all languages; even in languages that do have overt or pronounced relativizers, they do not necessarily appear all of the time. For these reasons it has been suggested that in some cases, a "zero relativizer" may be involved, meaning that a relativizer is implied in the grammar but is omitted in speech or writing. For example, the word that can be omitted in the above English example, producing "I have one you can use", using a zero relativizer.

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.