camber (of an airfoil)
The camber is the curve of the upper and lower surfaces of an airfoil. This curve is measured by how much it departs from the chord of the airfoil (the horizontal line joining the leading and trailing edges). Some airfoils have very little camber, i.e., the airfoil looks flat, while others have a higher degree of camber.
The term upper camber refers to the camber of the upper surface of the airfoil. The term lower camber refers to the camber of the lower surface of the airfoil. When the curve is away from the chord, the camber is said to be positive. When the curve is toward the chord, the camber is said to be negative.
The camber of an airfoil causes an increase in velocity and a consequent decrease in pressure of the stream of air moving over it.