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 dependent clauses
subordinate clause
noun
1.
(grammar) a clause with an adjectival, adverbial, or nominal function, rather than one that functions as a separate sentence in its own right Compare coordinate clause, main clause