Plain English Meaning
How much of the airplane's body you would see if you looked at it directly from the side. The bigger that side profile, the more the air pushes on it when the airplane is not flying perfectly straight into the wind.
Full Definition
The total surface area of the fuselage as viewed from the side of the aircraft, which influences directional and lateral stability by acting as a weathervaning surface in sideslips and crosswinds.
Context Anchor
Seen in discussions of airplane stability, especially how wing sweepback, wing location, and the shape of the airplane body affect the airplane after it is disturbed from straight flight.