a clause which qualifies or restricts the meaning of the noun in a noun phrase. It may be introduced by words such as who, which and that in English: the man who I saw this morning, the woman (that) I sent the letter to.
a dependent clause in apposition with a substantive for the purpose of specifying it. For example, "who works for my father" is the relative clause in the sentence "The man who works for my father goofed." The fact that my father is my relative has nothing to do with it.
Another name for adjectival clause. A subordinate clause which gives more information about someone or something mentioned in the main clause. E.g. The man, who wears a red shirt, is a famous comedian.
A relative clause is a subordinate clause that modifies a noun. For example, the noun phrase the man who wasn't there contains the noun man, which is modified by the relative clause who wasn't there. In many languages, relative clauses are introduced by a special class of pronouns called relative pronouns; in the previous example, who is a relative pronoun.