under-cambered wing
Most common airfoils, a vertical cross section of the wing, have a curved surface. There is always a curved top surface. Sometimes the bottom surface has the same curve as does the top surface, but as a mirror image (a symmetrical airfoil). Sometimes the bottom is just a straight line (a flat bottom airfoil). Sometimes, especially on gliders and most WW I aircraft, the bottom curve matches the top curve with just some space in between. This is an under-cambered airfoil. It produces the most lift but also the most drag.
