Answer:
A STALL, where the wing exceeds it's maximum angle of attack for the conditions (speed, g-force, etc>) under which the airplane is flying. Air separation over the top of the wing increases as angle of attack (angle of the wing compared to the airflow) increases, until the separation causes a decrease in lift. When under these conditions the amount of lift is less than the weight of the aircraft, the wing is stalled, and the aircraft descends. Since most aircraft don't seem to have an angle of attack meter, the airspeed indicator is usually marked with stall speeds experienced under various specific circumstances.