| Definition -
A subordinating conjunction that links a relative clause to its head noun.
1. It differs from a relative pronoun in that it doesn't have a nominal function within the relative clause.
2. Not all languages use them: Semitic languages do, most Indo-European languages use relative pronouns instead, and others, such as Japanese, use word order to indicate relative clauses.
Example - In the Hausa phrase na ga mutumin da ya yi aikin, the da is a relativizer. It translates to I see the man RELATIVIZER he do the work (i.e. I saw the man who did the work.)
Note: Despite translating it as who, da is not a relative pronoun: it is neither a pronoun, nor is it the subject of the relative clause.
Oxford English Dictionary - The word's first citation is from 1976:
"Conditions of use of the relativizer morphemes."
(Language & Lang. Behavior Abstr. X. 671/2)