A stall, in aviation, is a reduction in lift created by an airfoil and is caused by excessive angle of attack. This is most often at low speeds. As speed decreases, the angle of attack required to generate the same amount of lift increases; however, at the critical angle of attack, any increase in angle of attack will actually cause a reduction in coefficient of lift. The best remedy for a stall is to point the nose of the airplane downward, which decreases the angle of attack.
Because of this relationship between speed, angle of attack, and lift, specific "stall speeds" for given aircraft configurations exist and are commonly used for reference. These stall speeds are further effected in maneuvering flight. As an aircraft turns, it must generate more lift at a given speed to maintain altitude, thus at a given speed, angle of attack increases. This increases the "stall speed."
The Continental Connections Flight 3407 crash of February 12, 2009, is thought to have been caused in part by the captain's incorrect response to stall indications in the Q400 Bombardier. "Pulling up is the wrong thing to do when the airplane is giving you a stall warning," said William Waldock of the crash, a professor of safety science at Embry Riddle Aeronautical University who has investigated accidents for more than 30 years. "You increase the airspeed by pushing the nose down. He's pulling up fairly abruptly. There's an old adage that airspeed is life."