Definition
The path of an aircraft's flight as projected straight down onto the surface of the earth. It is the actual line traced over the ground, regardless of the direction the aircraft's nose is pointed.
Plain English
The line you would draw on the ground directly beneath the airplane as it flies. It shows where the airplane is actually going across the earth, not where its nose is pointing.
Context Anchor
In a crosswind takeoff and initial climb, you use ground track to judge whether the airplane is staying on the runway's straight-ahead path after liftoff or drifting to one side.
Derivation
Plain English: 'ground' (the surface of the earth) plus 'track' (a path or trail). The ground track is literally the trail the airplane would leave on the ground if it were dragging a pen straight down.
Why Pilots Care
Maintaining the proper ground track keeps the airplane aligned with the runway and on the intended route despite crosswind drift.
Analogy
Think of a boat crossing a river while pointing slightly upstream. The boat's nose points one way, but the boat actually moves diagonally across the water. The diagonal line across the riverbed is its ground track.
Intuition Check
Do not read ground track as the direction the airplane is pointed. Ground track means the path the airplane is actually making over the ground.
Example Sentence 1
After lift-off in a crosswind, the pilot crabbed into the wind to keep the ground track aligned with the extended runway centerline.
Example Sentence 2
The pilot checked the GPS to confirm the airplane was still on the desired ground track despite the wind.