A
particle from space that has fallen to the
earth.
To improve on the Webster 1913 definition I'd like to mention that the word meteorite is not interchangeable with meteor or meteoroid.
A meteoroid is moving in space. It becomes a meteor if it enters the Earth's atmosphere.
A meteor only becomes a meteorite if it survives the fast and fiery trip through the atmosphere and comes to rest on or impacts the Earth's surface. Of course it has to be found and identified by someone too, otherwise, it's just a rock.