a complex amine, CHNO, found in blood, the brain, etc. or produced synthetically: it constricts the blood vessels and contracts smooth muscle tissue, and is important both as a neurotransmitter and a hormone
See serotonin in American Heritage Dictionary 4
(sĕrˌə-tōˈnĭn, sîrˌ-)
noun
An organic compound, C10H12N2O, formed from tryptophan and found in animal and human tissue, especially the brain, blood serum, and gastric mucous membranes, and active as a neurotransmitter and in vasoconstriction, stimulation of the smooth muscles, and regulation of cyclic body processes.