Plain English Meaning
The amount of an airplane's side surface that sits below its balance point. When the airplane is pushed sideways through the air, this area catches the air and helps the airplane straighten itself out, much like the underside of a sailboat keeps it tracking straight in the water.
Full Definition
The total side area of an aircraft below its center of gravity, including the fuselage sides, vertical fin, and other vertical surfaces, that acts like the keel of a ship to provide directional and lateral stability when the aircraft is sideslipping or yawing.
Context Anchor
Seen in discussions of keel effect, weight distribution, and why some airplanes more naturally stable in than others.