Plain English Meaning

The bottom of the wing — the part you'd see if you stood under the airplane and looked up.

Full Definition

The lower surface of the wing — the side that faces the ground in normal flight. It is the surface opposite the upper (cambered) surface and is generally flatter, forming the lower boundary of the airfoil shape.

Context Anchor

You will see this phrase in explanations of how flaps change airflow around the wing and increase lift at lower speeds.

Why Pilots Care

Flaps mounted here increase both lift and drag, directly affecting approach speed, landing distance, and stall behavior.