mel·a·to·nin (m?l??-t??n?n)n. An animal hormone, C13H16N2O2, derived from serotonin and produced by the pineal gland. It stimulates color change in the skin of amphibians and reptiles and plays a role in regulating circadian rhythms and reproductive cycles in mammals. It is also found in plants and fungi.[Greek mel?s, black + ton(e) + -in.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.melatonin (?m?l??t??n?n) n (Biochemistry) the hormone-like secretion of the pineal gland, causing skin colour changes in some animals and thought to be involved in reproductive function[C20: probably from mela(nocyte) + (sero)tonin]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014mel?a?to?nin (?m?l ??to? n?n) n. a hormone secreted by the pineal gland in inverse proportion to the amount of light received by the retina, important in regulating biorhythms. [1955?60;