Considering how much overlap in genres, it is very tricky to come up with an agreed upon definition.
I don't consider Cadaver a platform game at all, even tho it has platforms.
Lemmings? I don't think of it as a platform game, but the movement between platforms is key to that game...
UGH.. It certainly looks very platformy.. And movement between platforms is crucial..
But I think, as you spend more time in the air than on the platforms, I'd say not... It would be more like Joust in that respect.
So, having platforms doesn't make a game a platform game..
But what about games where being on and moving between platforms is key to the game, as in Benefactor or Leander??
My initial reaction to those would be to consider them platform games, but it's been suggested otherwise..