I've always been into MMOs, ever since I laid my hands on Ultima Online. I always wanted to improve, discover my potential, learn as fast as possible. I've always been that kind of guy, in game and in life.
All of that has absolutely nothing to do with how lost I can get in a virtual world like WoW. Not so much in recent years, but earlier on, it was so easy to become blissfully overwhelmed by the absolute size of the game. I really felt as though I were in a living, breathing world. I wasn't logging in to raise stats or improve my gear, I was logging in to another world. To me, that's immersion. I was immersed in the world, not just a game. The same happens when I watch a good movie/series or read a good book.
The way I used to explain it to my friends was that I had 2 types of friends who played the game: One that played the game to play a game and the other type who played the game to escape reality for a few hours.
It's the latter that you generally find using the word "Immersion."