The important distinction is that you can not fly in anything that doesn't get out of the way as you move through it.

So the 'wheels on the ground' don't have anything to do with flight (though technically they are also in the air). Flight takes place in a medium. For a plane that is only one medium unless you're looking at a flying boat at take-off, for a sailing vessel there are two, there is the water (dense, slow to push out of the way so high drag) and the air (much less dense, so faster to push out of the way, much lower drag).