Plain English Meaning

The bottom side of the wing — the part you see when you look up at an airplane flying overhead.

Full Definition

The lower surface of an airplane wing — the side that faces the ground in normal flight. It is the boundary along the bottom of the wing where the airflow passes underneath, and it is the surface to which devices such as flaps, fairings, and inspection panels are typically attached.

Context Anchor

Seen in flap effectiveness discussions, especially when describing how air flows under the wing and around extended flaps.

Derivation

‘Under’ means below, and ‘surface’ means the outer face of something. Together the term simply names the lower outer face of the wing. Aviation uses it to clearly distinguish the bottom of the wing from the top (upper surface), because aerodynamically and structurally the two sides behave very differently.