Plain English Meaning

VSO is the slowest speed at which the airplane can still fly when it is fully set up to land — flaps down, gear down, and power off. Below this speed, the wing stops producing enough lift and the airplane stalls.

Full Definition

VSO is the calibrated stalling speed, or minimum steady flight speed, at which the airplane is controllable in the landing configuration — typically with full flaps extended, landing gear down, power at idle, and at the most rearward allowable center of gravity. It is shown on the airspeed indicator as the lower end of the white arc.