Definition
A reference airspeed equal to 1.3 times the airplane's stalling speed in the landing configuration (gear and flaps extended, idle power). It is the standard target final approach speed used to ensure a safe margin above the stall while on approach to landing.
Plain English
It's the speed you fly on final approach. You take the speed at which the airplane would stall with gear and flaps down, and multiply it by 1.3. That gives you a comfortable cushion above the stall as you come in to land.
Context Anchor
Seen in descent, approach, and landing discussions when choosing a safe target speed before touchdown.
Derivation
VSO comes from V (velocity) plus S (stall) plus O (the landing configuration -- gear and flaps fully extended). The 1.3 multiplier is the regulatory safety factor: 30 percent above stall speed, a margin chosen to keep the airplane comfortably clear of a stall during the approach.
Why Pilots Care
Using this speed provides a safe margin above stall during the final descent while keeping the airplane slow enough for a controlled touchdown.
Intuition Check
1.3 VSO is not the stall speed itself. It is a target speed above the stall speed for the airplane in its landing setup.
Example Sentence 1
On final approach, the instructor reminded the student to stabilize the airplane at 1.3 VSO before crossing the runway threshold.
Example Sentence 2
On a gusty day the pilot added five knots to 1.3 VSO for extra safety during the approach.