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. [ http://openisbn.com/search.php?q=0198506732&isbn=1/ http://openisbn.com/search.php?q=0781735149&isbn=1/ GOC:ef ]