outboard motor

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

• noun A detachable engine mounted on outboard brackets or on the stern of a boat.

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

• noun A detachable and pivotable propulsion unit fitted over the transom of a boat, comprising an engine that drives a propeller.

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

• noun internal-combustion engine that mounts at stern of small boat