Immersion is a seamless blend of graphics, audio design, controls and story that can suck you in for hours on end and make it seem like minutes.
STALKER is a very immersive experience, and probably makes the best use of my adrenal glands.
Fallout 3 had me nearly pass out from hunger after a marathon session.
Nazi Zombies, though wholly unrealistic, had me dreaming of the undead ripping into my house as I ran around boarding up windows.
Half Life 2 was interesting enough that my best friend's wife (who is NOT into video games) sat down and started asking questions about the Combine and the game world in general.