From IFWiki

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.