Plain English Meaning
A wing or blade shape where the top and bottom are the same. It only makes lift when tilted into the airflow, and it works the same way right-side up or upside down.
Full Definition
An airfoil that has identical curvature on its upper and lower surfaces, so the shape above the chord line is a mirror image of the shape below it. Because of this symmetry, it produces no lift at zero angle of attack and generates lift equally well in upright or inverted flight.
Context Anchor
Seen in discussions of wing sections, control surfaces, stabilizers, propeller or rotor blades, and airframe design.
Derivation
From Greek symmetria, meaning 'same measure' — the upper and lower surfaces have the same shape. 'Airfoil' refers to the cross-sectional shape of a wing or blade. Together: a wing-shape with halves above and below the .