Definition
An airfoil is a surface designed to produce a useful aerodynamic reaction — primarily lift — when it moves through the air. Wings, propeller blades, horizontal and vertical stabilizers, elevators, ailerons, and rudders are all airfoils. The shape is typically curved on top and flatter on the bottom, with a rounded leading edge and a tapered trailing edge, so that air flowing around it generates a pressure difference between the upper and lower surfaces.
Plain English
An airfoil is any specially shaped surface on an airplane that uses moving air to push or pull the airplane in a useful direction — most often, to hold the airplane up.
Context Anchor
Seen in stall awareness when discussing how a wing makes lift and how that lift decreases when the wing meets the airflow at too high an angle.
Derivation
From 'air' plus 'foil,' where 'foil' originally meant a thin sheet of metal. The word was adopted in early aviation to describe a thin, shaped surface designed to work with air — which captures exactly what a wing or control surface is doing.
Why Pilots Care
The airfoil shape determines the amount of lift generated and the angle at which the wing stalls, directly affecting aircraft performance and safety.
Intuition Check
Do not think of an airfoil as only a complete wing. The wing has an airfoil shape, but propeller blades and tail surfaces can also be airfoils.
Example Sentence 1
As the airfoil meets the oncoming air, the pressure difference between its upper and lower surfaces produces lift.
Example Sentence 2
During slow flight practice the instructor pointed out how the airfoil begins to lose lift as the nose rises too high.