A neurotransmitter; a chemical messenger that sends messages to our brain.
Scientists don’t agree yet on how the dopamine affects an ADHD brain; some say the brain has lower levels of it, others say is not properly handled, etc.
See The Role of Dopamine for a quick and easy understanding.
Lower Levels of Dopamine
Volkow et. al (2011)
“Decreased activity of the reward system in individuals with ADHD is likely to translate into problems in engaging in activities that are not inherently rewarding or reinforcing (and therefore may be described as less interesting or less motivating).”Volkow et al (2011) “Motivation Deficit in ADHD is Associated with Dysfunction of the Dopamine Reward Pathway” p. 7