I generally agree with SomethingBig. To me an RPG involves the growth of character not specifically plot wise, but having some system along the lines of using levels or some other form of it that allows the characters to grow and gain new abilities an' such.

I generally don't agree with the terms like Action-RPG or Adventure-RPG. Personally, I think those two are usually the same. I'd define Zelda as hack'n'slash action/adventure. Link gets new items'n'such, but it's not the same as gaining a level and getting a new ability.