“Highly restricted, fixated interests that are abnormal in intensity or focus” says the DSM 5. The most common term for these among autistic people and their families is “special interests.” Lots of people have hobbies; autistic people’s hobbies get special attention for two reasons: first, we often focus on them really intensely, and pursue them in unusual ways— wanting to learn everything there is to know about a topic, for instance, rather than wanting an activity to do. Second, we sometimes pick topics that other people consider obsucre, or silly, “Why would anyone be interested in that?” sort of things. Some autistic people focus on one topic for years, others may switch often or have several at once. For some reason the archetypal special interest is trains.
Julia Bascom wrote a wonderful essay about how having a special interest feels, called “The Obsessive Joy of Autism”, which you can read here.
As for me, I don’t tend to focus on one interest really intensely, but I do have a special-interest-ish way of going about a lot of my interests. I can spend hours reading though forums and user guides and wikis for my favorite computer game… instead of actually playing the game. Sometimes I read information about games I don’t own and never even intend to play, like the descriptions and vital statistics of DnD monsters. (Small evil ants. Medium-sized evil ants. Giant evil ants. The spell you can use to summon evil ants, and what level you need to be to get each size. etc. etc. etc. To me, this is a lovely, soothing progression of information.) I can also spend hours using Ravelry’s knitting pattern search function, finding the perfect pattern for the hypothetical hat or sweater I’ll probably never make. I find reading and sorting and collecting information to be soothing.
I do feel the obsessive joy that Julia Bascom talks about. I have a whole host of science and history facts that are so! cool! which I try to avoid boring people with.
That’s another autistic trait, by the way– having one all-consuming interest + having some difficulty with social interaction = “infodumping” lots of facts that are fascinating to you but not interesting to your conversational partner. I’m pretty good at controlling this urge, I think, but I definitely have it.