a clause that modifies the principal clause or some part of it or that serves a noun function in the principal clause, as when she arrived in the sentence I was there when she arrived or that she has arrived in the sentence I doubt that she has arrived.
British Dictionary definitions for subordinate clause
subordinate clause
noun
grammara clause with an adjectival, adverbial, or nominal function, rather than one that functions as a separate sentence in its own rightCompare coordinate clause, main clause
