Immersion is the feeling of being involved within the world of the game. Things that "break immersion" are those that remind the player (often undesirably) that he is playing a game, rather than interacting with a story or environment.
The term immersion is often used semi-interchangeably with mimesis. Things that break mimesis are, from a game-design point of view, sometimes considered flaws because they also break immersion; but not everything that destroys immersion is itself a flaw in mimesis. Bad user interface choices and other problems may also distance the player from immersion in the game.