Plain English Meaning
The slowest speed the aircraft can fly without stalling when it is set up to land, with the wheels and flaps down.
Full Definition
The minimum airspeed at which an aircraft can maintain level flight in the landing configuration — gear down and flaps fully extended — at maximum gross weight. It is represented by the lower end of the white arc on the airspeed indicator and is designated VS0.
Context Anchor
Seen in airspeed indicator color-code discussions, aircraft manuals, and landing-speed planning.
Derivation
“Flap” refers to a movable part on that can be lowered to change how the wing produces . “” in means the wing is no longer producing smooth, usable lift because the it has down. Together, “flaps-down stall speed” points to the with the lowered.