contractor

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

• noun One that agrees to furnish materials or perform services at a specified price, especially for construction work.
• noun Something, especially a muscle, that contracts.

from The Century Dictionary.

• noun One who contracts; one of the parties to a contract, bargain, or agreement; one who covenants with another to do or to refrain from doing a particular thing.
• noun Specifically One who contracts or covenants, either with a government or other public body or with private parties, to furnish supplies, or to construct works or erect buildings, or to perform any work or service, at a certain price or rate: as, a paving-contractor; a labor-contractor.
• noun A muscle which contracts or lessens the size of a part; a constrictor.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who contracts; one of the parties to a bargain; one who covenants to do anything for another; specifically, one who contracts to perform work on a rather large scale, at a certain price or rate, as in building houses or making a railroad.

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

• noun A person who executes the building or improving of buildings.
• noun A person or company that performs specific tasks like electrical or plumbing work in construction projects.
• noun A person or company hired to maintain existing facilities like air conditioning systems, groundskeeping, etc.

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

• noun someone (a person or firm) who contracts to build things
• noun a bodily organ that contracts
• noun (law) a party to a contract
• noun the bridge player in contract bridge who wins the bidding and can declare which suit is to be trumps