Plain English Meaning
The slowest speed at which the airplane can still fly in a controlled way when it is set up to land — flaps fully out, gear down, and power at idle. Below this speed, the wing stops producing enough lift and the airplane stalls.
Full Definition
The 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, and power at idle. VS0 marks the lower end of the white arc on the airspeed indicator.
Context Anchor
Seen in aircraft speed , , and discussions of stalls and landing configuration.