Constituent (linguistics)
Phrases (noun phrases, verbal phrases, etc.) are usually constituents of a clause, but clauses may also be combined into a bigger structure.  For example, in the phrase "I didn't hear what you said," the subordinate clause "what you said" is combined into the main clause and is syntactically its object; this can be demonstrated by substituting the pronoun "it" for the subordinate clause "I didn't hear it."