A dependent clause is a °clause that cannot occur on its own in non-elliptical contexts.
All subordinate clauses are dependent, but not all dependent clauses are subordinate. A form that cannot occur on its own is a bound form, so one could also say that a dependent clause is a "bound clause".
dependent clause (CXN) = a clause that is a dependent of a matrix clause in a complex sentence construction. Adverbial clauses, complement clauses, and relative clauses are all examples of dependent clauses. Example: in She watered the plants before she ate lunch, before she ate lunch is an instance of an (adverbial) dependent clause. Dependent clauses are often, but not always, subordinate clauses. (Section 15.1.2)