Plain English Meaning
The slowest speed the airplane can fly in landing configuration — gear down, flaps down, power off — before it stalls. It's the bottom of the white arc on the airspeed indicator.
Full Definition
The calibrated stalling speed, or the minimum steady flight speed at which the airplane is controllable, in the landing configuration — meaning gear down, flaps fully extended, power at idle, and at the most forward center of gravity. On the airspeed indicator, VSO is marked by the lower end of the white arc.
Context Anchor
Seen in landing, descent, , and airspeed indicator discussions; it is often the lower end of the white arc on the airspeed indicator.