My definition...
I got the term from "Wayne's World," and took from the context at the time the idea of a person not only intensely obsessed about something--usually something very specific--but often quite enthusiastic about this obsession. You will never find such a person leaving the field for the sports bar. This person will be out on the fifty yard line after the game is over and the lights are out, reliving the last play or planning the next one.
In later years, I found out a more accurate definition seems to refer to women who are grossly possessive, paranoid, and clinging, but I like my usage better, and can apply it more widely.