When discussing roleplaying games I’ve tried to eliminate the term “immersion” from my vocabulary: It’s terminology with a horribly fractured etymology and never fails to create confusion whenever it’s used.
The problem has its primary roots in the ’90s: In the tabletop community, the Usenet groups picked the term “immersion” to refer to people deeply immersing themselves in the playing of their character. “Deep immersion” became the state in which roleplaying flowed naturally and you were able to make decisions as your character an portray your character without have to engage in logical analysis.
Almost simultaneously, however, the video game community created the concept of “immersion vs. interactivity”. In this construct, loosely speaking, interactivity refers to the player making decisions and immersion refers to the player becoming drawn into or convinced by the faux reality of the game world. (You’ll notice that, in this construction, the concept of “immersion” is effectively set up as being in a state of antithesis with the tabletop community’s use of the word “immersion”.) This video game concept of “immersion” then “jumped the pond” and got picked up by various tabletop communities.
Then you can take all of that confusion and stir in a healthy dose of people using the word according to its general dictionary definition: “Deep mental involvement.” That meant any time somebody said “no, immersion is about deep mental involvement in X” (whether X was “playing your character” or “the presentation of the game world”), somebody else could respond by saying “no, I experience immersion by having a deep mental involvement with Y”.
My personal use of the term was shaped in those old Usenet discussions. So if you ever do see me using the word “immersion” in the context of tabletop roleplaying, it’s a virtual certainty that I’m talking about immersion in the process of roleplaying a character; the sort of one-to-one flow of thought to action and the empathetic flow of thought that often characterizes our conception of the very best Method actors. But I’ve generally found that when I need to discuss that sort of thing it’s almost always more rewarding to find a way of talking about it which doesn’t use the word “immersion”.
Whatever your personal conception of the word “immersion” is, I recommend you do the same.