I would say, anything whose spacetime geodesic (orbital/freefall path) intersects the spheroid defined by the surface of the Earth. Though by this definition, a comet on a 100-year collision course is already “on Earth”, so I’m not sure if that’s reasonable.
https://knowyourmeme.com/memes/family-laughing-at-crying-child-opening-christmas-present
That includes some history, but not the prompt itself.