Autistic slang for an object that is fun to stim on, particularly a sensory stim. Perhaps because of the fact that we commonly have narrowly focused minds and unusual sensory wiring, autistic people can often derive a lot of pleasure out of a simple object. It may be an ordinary household object taken out of context, like a key ring or a bicycle (for spinning the wheels, not for riding). It may be something specifically designed for even neurotypical people to stim on, like a koosh ball or spinning top. It may be a piece of reflective, prismatic paper that most people would find interesting or pretty but that an autistic person might stare at all day.
Common stimtoys are shiny, spin, move in unusual ways, or have a certain texture.
Other people have come up with their own names for similar concepts. Since LSD appears to mimic a few aspects of autistic senses, it's inevitable that some tripper out there would come up with the concept of trip toys. People with attention deficit disorder have ADD toys. Geeks have geek toys. Desk toys are even prevalent in the offices of neurotypical people.
While any of these could be used as a stimtoy, stimtoys also extend to objects that most neurotypicals would rarely stim on, like a piece of string or tinfoil. Also, autistic people tend to have a very long attention span for their favorite stimtoys. Familiar stimtoys can double as a source of comfort when disoriented in unfamiliar environments.
Example usage: "I've got a koosh ball, but I'd prefer a visual stimtoy right now."