What characteristic defines a positive camber airfoil?

A positive camber airfoil is characterized by a convex curve on its upper surface, which is instrumental in generating lift. The shape of the airfoil, particularly the curvature of its upper surface, plays a significant role in how air moves over and under it. When the airfoil's upper surface is convex, the air traveling over it moves faster than the air beneath the lower surface, which is typically flatter. This difference in velocity creates a pressure difference – lower pressure on top and higher pressure underneath – thereby resulting in lift.

In the context of the other possible answers, it's noted that while the concept of camber involves curvature, a concave design on the lower surface or a straight lower surface does not define positive camber but rather neutral or negative camber configurations. Therefore, the defining feature of a positive camber airfoil is indeed the convex curve on the upper surface, which enables effective lift generation during flight.