The regulated release of serotonin by a cell. Serotonin (5-hydroxytryptamine, or 5-HT) is a monoamine synthesised in serotonergic neurons in the central nervous system, enterochromaffin cells in the gastrointestinal tract and some immune system cells. [ https://www.worldcat.org/search?q=bn%3A0198506732 https://www.worldcat.org/search?q=bn%3A0781735149 ]