Immersion is a process by which your senses and actions get redirected along pathways that extend you. Controlling a game doll is like driving a car or operating an RC plane. The difference with a video game (or any game) is that the immersion is happening in another place with its own rules rather than this one.
Immersion has two components: projection (how the player is extended into the game world) and transference (how effects within the game world provoke physical or basic emotional responses in the player).