Definition
VSO is the calibrated stalling speed, or the minimum steady flight speed at which the airplane is controllable, with the aircraft in the landing configuration — meaning gear down, flaps fully extended, power at idle, and the airplane at maximum landing weight. It is published in the Pilot's Operating Handbook and is marked on the airspeed indicator as the lower end of the white arc.
Plain English
VSO is the slowest speed the airplane can fly without stalling when it is set up to land — wheels down, flaps all the way out, and power pulled back. It is the bottom edge of the white band on the airspeed indicator.
Context Anchor
Seen in aircraft manuals, performance charts, airspeed indicator markings, and discussions of approach and landing speeds.
Derivation
The 'V' comes from the French 'vitesse,' meaning speed — the international convention for airspeed designators. The 'S' stands for stall. The 'O' is short for 'in the landing configuration' (sometimes remembered as 'zero flap setting position for landing,' though it specifically means the configuration with gear and flaps fully out for landing). Knowing 'V' means speed helps make sense of the whole family of V-speeds (VS, VSO, VS1, VFE, VNE, and so on).
Why Pilots Care
It sets the minimum safe speed for the landing approach and directly affects landing distance and stall margin calculations.
Intuition Check
Minimum does not mean a recommended speed to fly on approach. It means the published lower limit for steady flight in the landing configuration; normal approach speeds are higher.
Example Sentence 1
On final approach, the instructor reminded the student to fly at 1.3 times VSO to keep a safe margin above the stall.
Example Sentence 2
With the aircraft at maximum landing weight, VSO increased by three knots compared with the lighter training flights.