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.
Plain English
How fast the aircraft is actually moving across the ground below it, taking the wind into account.
Context Anchor
Seen on navigation logs, flight planning forms, GPS displays, and IFR enroute planning when calculating time between points.
Derivation
Plain English: 'ground' (the earth below) plus 'speed' (rate of travel). The phrase exists to distinguish this measurement from airspeed, which is the aircraft's speed through the air mass — not the same thing when wind is blowing.
Why Pilots Care
Determines estimated time en route and fuel consumption.
Grounding Statement
If the airplane is moving through the air at 120 knots but has a 20-knot headwind, it is only covering the ground at about 100 knots.
Intuition Check
Do not assume ground speed is the same as airspeed. Ground speed is what the airplane is doing over the ground after wind is taken into account.
Example Sentence 1
With a strong headwind on the leg to the next waypoint, the ground speed dropped to 95 knots and the pilot revised the estimated time of arrival.
Example Sentence 2
A headwind reduced the aircraft's ground speed during the approach.