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.
Plain English
The bottom of the wing — the part you'd see if you stood under the airplane and looked up.
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.
Intuition Check
Do not think of this as a separate part bolted onto the wing. Here, it means the lower surface of the wing itself, especially as it matters to airflow and flap operation.
Example Sentence 1
The flap is hinged to the underside portion of the wing near the trailing edge.
Example Sentence 2
The increased camber on the underside portion of the wing produces more lift at the same airspeed.