Immersion: sinking stuff in water.
When speaking of computer games and other forms of media, "immersion" could be translated as "ability of media to make the subject forget that what she sees and hears is not real".
For example, an immersive computer game should be absolutely realistic; If illogical or weird things happen in game, the "illusion" is broken.
Immersion, computer-game-wise, doesn't necessarily imply graphic details and good sound; Nethack is definitely an immersive game, even when the graphics are not that great.