Serotonin: A hormone, also called 5-hydroxytryptamine, in the pineal gland, blood platelets, the digestive tract, and the brain. Serotonin acts both as a chemical messenger that transmits nerve signals between nerve cells and that causes blood vessels to narrow.

Changes in the serotonin levels in the brain can alter the mood. For example, medications that affect the action of serotonin are used to treat depression.