Angle of attack
In the diagram above, the black arrow represents the direction of the wind. The wing is shown end on. The angle α is the angle of attack.
Angle of attack is a term used in aerodynamics to describe the angle between the wing's chord and the direction of the relative wind, effectively the direction in which the aircraft is currently moving. The amount of lift generated by a wing is directly related to the angle of attack, with greater angles generating more lift. This remains true up to the stall point, where lift starts to decrease again because of airflow separation. Planes flying at high angles of attack can suddenly enter a stall if, for example, a strong wind gust changes the direction of the relative wind, an effect that is seen primarily in low-speed aircraft.
In military terminology, angle of attack is often referred to as alpha (α), the symbol used to denote it on most diagrams. Using a variety of additional aerodynamic surfaces (a.k.a. high-lift devices) like leading edge extensions, fighter aircraft have increased the potential flyable alpha from about 20 degrees to over 45, and in some designs, 90 degrees or more. That is, the plane remains flyable when the wing's chord is at right angles to the direction of motion.