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English definition of “serotonin”

serotonin

noun [U]     /ˌse.rəˈtəʊ.nɪn/ US  /-ˈtoʊ-/
a neurotransmitter (= a chemical in the body that carries messages from the brain ) that helps you feel relaxed and happy Animal biochemistry
(Definition of serotonin noun from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)
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