Angle of attack
From FlightSim
The angle of attack (sometimes abbreviated AoA) is the angle between the wing and the oncoming air, that is, it is the angle at which the wing "attacks" the air.
At low angles of attack, the air tends to flow smoothly over most of the wing. As the angle increases, a turbulent area (air no longer smooth -- eddies) starts building at the trailing edge of the wing. As the angle continues to increase, that turbulent area moves forward, until the critical angle of attack is reached (often in the neighborhood of 18ยบ or so), when there is no longer a large enough area of smooth air flowing over the wing, and the wing stalls, that is, it no longer has enough lift to support the aircraft. So the aircraft drops its nose to try to pick up flying speed again.
See also the Wikipedia entry angle of attack.
