a form of subordinate clause that attaches to nouns or noun clauses and  start with the relative pronouns who, that, which, whose, where, when. Relative clauses can be defining or non-defining:                                           

• A defining clause gives essential information about the noun or noun clause so that a sentence makes sense, e.g. The uncle who you met earlier is a lawyer.
• A non-defining clause gives extra information, which is not essential, about the noun or noun clause, e.g. The farmer, who is the kindest person, grows beans.