Definition
The speed of an aircraft measured relative to the ground over which it is flying. It is the airspeed adjusted for the effect of wind — a headwind reduces groundspeed, a tailwind increases it.
Plain English
How fast the aircraft is actually moving across the ground beneath it, regardless of how fast it is moving through the air.
Context Anchor
In the Glides chapter, groundspeed matters when judging how far the airplane will travel across the ground during a glide.
Derivation
A straightforward compound: 'ground' (the surface below) plus 'speed' (rate of motion). The point of the term is to distinguish this measurement from airspeed, which is measured relative to the air mass — and the air mass itself is usually moving.
Why Pilots Care
Directly determines how far an aircraft will travel during a glide and the time available to reach a suitable landing site.
Intuition Check
Do not confuse groundspeed with airspeed. The airplane can have the same flying speed through the air while moving slower or faster across the ground because of wind.
Example Sentence 1
With a 15-knot tailwind, the airplane's groundspeed was higher than its indicated airspeed, so it covered the leg faster than planned.
Example Sentence 2
A strong headwind lowered the groundspeed and reduced the glide range to the airport.