In a nutshell, monotropism is the tendency for our interests to pull us in more strongly than most people. It rests on a model of the mind as an ‘interest system’: we are all interested in many things, and our interests help direct our attention. Different interests are salient at different times. In a monotropic mind, fewer interests tend to be aroused at any time, and they attract more of our processing resources, making it harder to deal with things outside of our current attention tunnel.
Source: Me and Monotropism: A unified theory of autism | The Psychologist
Monotropism is promising as a unifying theory for both autism and ADHD. This list of 6 common features of autistic thinking applies also to ADHD. Autism and ADHD are interest-based operating systems.