It is entirely subjective of course.

When I can be entirely focused on a game and interacting with the game or things that happen don’t break that focus. Sometimes this can mean confortable controls, worlds that have natural barriers, and options to interact that cover what I am trying to do.

Limitations on interactions, characters being inconsistent, and finding it hard to do the things that I feel should be possible in the game are immersion breaking or may even keep me from being immersed. Introductions that are obviously telling you how the game works are not immersive, but if they feel like part of the game they can be immersive.

Helldivers 2’s boot camp is immersive because it feels like things you do in boot camp with a healing dose of in world propaganda. Expedition 33’s into was immersive because it was doing in workd things and barriers didn’t stand out even if the pathing was obviously restricted for game reasons because things were happening! Both games continue to be immersive, but they are also examples of games that are immersive from the first moment the game starts and they keep it up from then on.