A sail has a mast,a boom,a luff,a leech,a head and a clew.
It has a leach, it has a roach, the mast inflates, it doesn't need a head because it has two clews.
...a kite is no different than a spinnaker on a yacht...
A kite is a lifting-foil, a spinnaker isn't - they are very different. But they are both sails, as they both require pressure differential on both sides of the cloth.
The only point in question is whether the sail must have a solid attachment to the hull - and even that discussion is tenuous at best.