I always understood that an otaku related more or less to a "useless" field. Such as manga, anime, ramen or trains, which don't really contribute much to one's life, unless you stretch it quite a bit.

I mean you could make a profession, but they're quite rare. Cooking, sciences and so on, I always thought didn't really count as otaku material, because they can easily be transformed into jobs and are usefull skills.