Stimming is short for “self-stimulatory behaviours” and is specifically about things we do in order to get particular types of sensory input. It’s usually repetitive in nature, though not always.
The term was originally coined by professionals who worked with developmentally disabled people, which is a population that is highly likely to engage in stimming. This population includes autistic people; it also includes people who have Down syndrome, Fetal Alcohol Spectrum Disorder, and - believe it or not - ADHD. So this word is not an exclusive word that only autistic people can use. It is ours, too.
Stimming is very similar to fidgeting, and some fidgety behaviour is stimming.
Stimming is often used to regulate the body or emotions, and it can also be used to express emotions.
I have good grades, does this mean I can't have ADHD?
Everyone say it with me: academic performance is not an indicator of ADHD. Certainly it can be an indicator, since executive dysfunction and hyperactivity and impulsive behaviours can have a big effect on your ability to complete assignments and participate adequately in class. However, if you are also gifted (common), or if you have Inattentive ADHD, then you will often fly under the radar and do well (or at least not poorly) academically, even though you struggle in some areas.
What is the difference between hyperfocus, special interests, and hyperfixation?
As noted above, hyperfocus is a period of sustained focus, typically lasting for several hours. Special interests are things that you are really interested in, often to the point of obsession. A special interest can last for a day or even several years. Often you will hyperfocus on a special interest.
The term “hyperfixation” is a new term that is meant to be an umbrella term for things like hyperfocus, special interests, and perseveration. You can read more about it here.