define serotonin
Medical Definition of Serotonin. Serotonin: A neurotransmitter that is involved in the transmission of nerve impulses. Serotonin can trigger the release of substances in the blood vessels of the brain that in turn cause the pain of migraine. Serotonin is also key to mood regulation; pain perception; gastrointestinal function, including perception of hunger and satiety; and other physical functions.