Plain English Meaning

The slowest speed the airplane can fly without stalling when the gear is up and the flaps are up. Go any slower in this configuration and the wing stops producing enough lift to keep flying.

Full Definition

VS1 is the stalling speed, or minimum steady flight speed, of an aircraft in a specified configuration — typically with the landing gear and flaps retracted (the 'clean' configuration). Below this speed, the wing can no longer produce enough lift to support the aircraft in level flight at 1G, and a stall will occur.

Context Anchor

Seen on airspeed indicator markings and in information when discussing and the .