Plain English Meaning
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.
Full 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.
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.