An outboard motor is an engine mounted on the outside back edge of a boat, rather than built inside the boat's hull. You can recognize an outboard by its distinctive shape: it hangs off the stern (the back of the boat) with the propeller underwater and the engine above, and it usually tilts up when not in use.

Outboards revolutionized small boat design when they became popular in the early 1900s. Before outboards, small boats needed oars or sails, or they had heavy engines permanently installed inside. An outboard motor can be removed, repaired, or replaced without major boat surgery. A fishing boat might use a small outboard that one person can lift, while larger boats might have powerful outboards that require special equipment to move.

Outboard can also describe anything positioned outside a boat's sides or stern. Sailors talk about the outboard side of an anchor (the side facing away from the boat) or outboard fuel tanks (portable tanks kept outside the boat's main structure). The opposite is inboard, meaning inside or toward the center of the boat.