Plain English Meaning

The slowest speed at which the controls still 'bite' the air enough to steer the airplane. Go slower than that and the controls stop working well enough to keep the airplane pointed where you want it.

Full Definition

The minimum airspeed at which the airplane's flight controls remain effective enough to maintain directional control of the aircraft. Below this airspeed, the rudder, ailerons, and elevator no longer produce sufficient aerodynamic force to counter yawing, rolling, or pitching tendencies, and the pilot loses the ability to control the airplane's path through the air.