What are the differences/definitions of a 2d platformer, a 3d platformer, and a sidescroller?

Question

i am talking about in video games.

Answer 1

2D Platformer: Something handheld Wario games or Castlevania, where you can pretty much go anywhere (on a 2d playing field).

3D Platformer: Super Mario 64, Psychonauts, and pretty much any other platformer on nextgen consols. 3d environment + lots of jumping and puzzle solving.

Sidescroller: MetalSlug, New Super Mario bros and others spaceship shooters. Sidescrollers are usually 2D/pseudo 3D shooters.  

Answer 2

It is pretty simple a 2d platformer is in 2d, and a 3d platformer is in 3d. A side-scroller can only be a in a 2d game because you scroll (walk) from side to side.