Stall

From Pilots Almanac

A stall is a loss of lift experienced by an airfoil when the critical angle of attack has been exceeded. Stalls are not a function of airspeed, but rather of the angle at which the relative wind strikes the wing.

Smooth, layered airflow above and below a wing allow for the production of lift (primarily caused by the downward bending of airflow). This can occur at angles of attack up to 16-20 degrees. Exceeding this critical angle of attack, however, causes the airflow over the wing's upper surface to separate. This turbulent, burbling airflow above the wing reduces lift dramatically.

The only way to recover from a stall is to reduce the angle of attack.

In the images below, the relationship of angle of attack and production of lift is shown. As angle of attack increases, the coefficient of lift increases until the airflow above the wing is no longer able to remain attached to the wing's upper surface. At this point, the stall occurs and the coefficient of lift drops suddenly.