Definition
Ground speed is the actual speed of an aircraft over the ground. It is true airspeed adjusted for the effect of wind — increased by a tailwind component and decreased by a headwind component.
Plain English
How fast the aircraft is actually moving across the surface below it. If you watched the airplane from the ground, this is the speed you would see it traveling.
Context Anchor
You will see ground speed in GPS displays, flight planning, navigation logs, and arrival-time estimates.
Derivation
From 'ground' (the surface beneath the aircraft) and 'speed' (rate of motion). The phrase emphasizes that the reference is the earth below, not the air the aircraft is moving through.
Why Pilots Care
It determines arrival time, fuel burn, and whether the airplane can reach its destination with reserves remaining.
Analogy
Think of swimming in a river. Your effort through the water is your airspeed. How fast you're actually moving past the riverbank is your ground speed — the current either helps you or holds you back.
Intuition Check
Do not assume ground speed is the same as the number on the airspeed indicator. Wind can make the airplane move across the ground faster or slower than it is moving through the air.
Example Sentence 1
With a strong tailwind at altitude, the ground speed climbed to 165 knots even though true airspeed was only 140.
Example Sentence 2
The GPS showed a ground speed of 85 knots while we flew the approach into a headwind.