I don't believe any game has, or could define Zelda, because the series can be anything it wants to be. If the developers wanted to make a futuristic title, it would still be Zelda, because it shares that name. The great thing about this, is it leaves the doors open to countless possibilities. Take Metroid, for example. You compare the classic 2D games, the Prime series, and Other M; all of them are quite different from each other in numerous aspects, yet they are still Metroid.