I'm not as extreme as some.
(people viewing me from the outside may differ!)
"Special interest" is a more neutral term.
For some of us, yes, it can be an obsession that almost literally cannot be put down, a sole topic of conversation.
But for others other interests and topics exist, alongside an awareness that not everyone shares our enthusiasm for whatever it might be.
Well, for me:
Using English well, which comes with a spin-off delight in English absurdities and mistakes, and word-play humour.
Philosophy and cosmology: trying to find the "rules of the game" for life, the universe and everything
(Yes, Douglas Adams was a great help!)
That spins off into a love of history, and seeing how divergent popular understanding can be from detailed facts.
Oh, and model railways for light relief, but done in a peculiar way which has got me an international reputation in a tiny niche field.
Edit: I take it that the borderline to obsession has been crossed when an individual can't choose to put it down, or register any other sense of priorities.
"A fanatic is one who can't change his mind and won't change the subject." Sir Winston Churchill