Definition
The actual speed of an aircraft over the ground. It equals the aircraft's true airspeed adjusted for the effect of wind — increased by a tailwind component and reduced by a headwind component.
Plain English
How fast the airplane is actually moving across the land below it. If the wind is pushing you along, your groundspeed is higher than your airspeed. If the wind is pushing against you, it's lower.
Context Anchor
Pilots see GS during flight planning, navigation, and on many cockpit displays, especially when checking time to the next airport or checkpoint.
Derivation
A plain compound of 'ground' and 'speed' — the speed measured relative to the ground, as opposed to airspeed, which is measured relative to the air the aircraft is flying through.
Why Pilots Care
It determines time to destination, fuel burn, and whether you will arrive before conditions change.
Analogy
Walking on a moving walkway at the airport: your walking pace is your airspeed, but your groundspeed is how fast you actually pass the shops on either side. With the walkway, you cover ground faster; against it, slower.
Intuition Check
Do not confuse groundspeed with the airplane's speed through the air. Wind from behind can increase GS, and wind from ahead can decrease GS.
Example Sentence 1
With a 30-knot tailwind, the pilot noted a groundspeed of 150 knots even though the airspeed indicator showed 120.
Example Sentence 2
The pilot used groundspeed to calculate the time remaining to the next waypoint.