A subordinate clause is a °clause that is a °relative clause, a °complement clause, or an °adverbial clause.
See Haspelmath (1995) and Cristofaro (2003) for discussion of the notion of subordination (or embedding). Most complement and relative clauses are unquestionably subordinate, but for many adverbial clauses, this is not so clear, and some relative clauses ("correlative relative clauses") are not clearly subordinate either.