Plain English Meaning
How fast the aircraft is actually moving across the ground beneath it, regardless of how fast it is moving through the air.
Full 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.
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 — and the air mass itself is usually moving.