A stall is an aerodynamic condition wherein the angle of attack of a wing increases beyond the "critical angle of attack", causing the wing to cease generating lift.
A stall is an aerodynamic condition wherein the angle of attack of a wing increases beyond the "critical angle of attack", causing the wing to cease generating lift. It's important to note that stalls can happen at any airspeed and in any attitude; the only cause of a stall is exceeding the critical angle of attack.
As the angle of attack of a wing increases it generates more lift, up until the critical angle of attack. Increasing the angle of attack beyond the critical angle results in a turbulent, detached airflow along the top of the wing.
At this point the wing is no longer generating lift, and the angle of attack must be reduced to allow the wing to produce lift again.
For more information about aerodynamic stalls and recovery see the Wikipedia article.
