Plain English Meaning
The slowest speed the airplane can fly without stalling when it is in a specified clean configuration — usually meaning flaps and landing gear up.
Full Definition
VS1 is the stalling speed or minimum steady flight speed obtained in a specific configuration, typically with flaps retracted and the airplane in a clean (cruise) configuration. It represents the slowest speed at which the airplane can maintain controlled, level flight before the wing stalls in that configuration.
Context Anchor
Seen in airplane , , and discussions when comparing safe operating speeds for a given airplane setup.