Plain English Meaning
How fast the aircraft is actually moving across the ground below it, taking the wind into account.
Full Definition
Ground speed is the actual speed of an aircraft over the ground, measured in knots. It is the result of the aircraft's airspeed combined with the effect of wind. A tailwind increases ground speed; a headwind decreases it. Ground speed is the figure used to calculate how long it will take to cover a known distance over the earth.
Context Anchor
Seen on navigation logs, flight planning forms, GPS displays, and IFR enroute planning when calculating time between points.