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Stall speed is slowest speed a plane can fly to maintain level flight. Normally, when a plane slows down it makes less lift. If the plane tilts its wings up, also known as increasing its angle of attack, it can compensate for any lift lost. However, if it tilts its wings too far, the air pulls away from the top of the wing and the plane loses lift. This is known as a stall.

Stall speed can be reached by increasing the angle of attack as close to stall as possible and slowing down until weight and lift balance out. Because the stall speed is dependent on the airplane’s weight, altitude, acceleration, and other factors, airspeed indicators are used to predict stall conditions in airplanes.