Definition of melatonin noun from the Oxford Advanced Learner's Dictionary

 

melatonin

 noun
noun
BrE BrE//ˌmeləˈtəʊnɪn//
 
; NAmE NAmE//ˌmeləˈtoʊnɪn//
 
[uncountable](biology)
 
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a hormone that affects skin colour and is thought to be involved in controlling the reproductive cycle Word Origin 1950s: from Greek melas ‘black’ + serotonin.
See the Oxford Advanced American Dictionary entry: melatonin