endocentric

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Of or relating to a group of syntactically related words, at least one of which is functionally equivalent to the function of the whole group. For example, the noun table in the noun phrase the old table is endocentric because it has the same grammatical function as the whole noun phrase.
• adjective Of or relating to a compound word whose referent is the same as the referent of one of its constituent parts. For example, the noun blackboard is endocentric in that it refers to a type of board.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective grammar fulfilling the same grammatical role a one of its constituents e.g. the noun "houseboat" because "boat" is also a noun

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective fulfilling the grammatical role of one of its constituents