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.
Plain English
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.
Context Anchor
You see VSO in the airplane handbook, on airspeed markings, and when managing speed on base leg and final approach.
Derivation
The 'V' comes from the Latin velocitas, meaning speed. The 'S' stands for stall. The 'O' is shorthand for the landing configuration — historically thought of as 'flaps out' or the configuration with everything extended. Knowing the letters stand for 'velocity, stall, landing configuration' helps separate VSO from VS1 (stall speed in a specified clean configuration).
Why Pilots Care
Pilots multiply VSO by 1.3 to set a safe approach speed that keeps a proper margin above stall during landing.
Grounding Statement
VSO is the bottom-end speed reference for controlled flight when the airplane is configured to land.
Intuition Check
VSO is not the recommended speed to fly on approach; it is a stall-speed reference. Landing configuration does not mean merely planning to land; it means the airplane is physically set up for landing.
Example Sentence 1
The instructor reminded the student that final approach speed should be flown at about 1.3 times VSO to provide a safe margin above the stall.
Example Sentence 2
The pilot's operating handbook lists VSO as 52 knots for this airplane at maximum weight.