Usually refers to the angle between the oncoming air and the wing.
Angle of Attack (AoA) is the angle between the oncoming air or "relative wind" and some reference line on a structure. For an airfoil, the most common reference line is a line from the foremost point on the leading edge to the aftmost point on the trailing edge.
For highly cambered airfoils (or airfoil + flaps) this line may exist partially outside the structure.
Angle of Attack is very useful in predicting aerodynamic stalls, coefficient of lift, etc.
