Track limits
The white lines define the edge of the track, and a driver has left it when all four wheels are beyond the line.
The all-four-wheels test is what makes it objective, and it is why replays linger on a single tyre touching white paint. Kerbs outside the line do not count as track.
In qualifying, a lap that exceeds them is deleted. In a race it is usually managed by warnings, with a five-second penalty once a driver has collected enough of them — which is what the black and white flag is often signalling.