I believe a continent is defined by whether it's on its own tectonic plate. Asia and Europe are on two separate plates that smashed into each other creating the Ural mountains. That's why they're considered different continents. Australia is on it's own plate with Oceana and New Zealand, making it a continent. Greenland, however, is on the same plate as the rest of North America, so it's just an island. India is apparently on it's own plate as well, but it's so small that it's just called a sub-continent.