noun. An organic compound, C₁₀H₁₂N₂O, formed from tryptophan and found in animal and human tissue, especially the brain, blood serum, and gastric mucous membranes, and active as a neurotransmitter and in vasoconstriction, stimulation of the smooth muscles, and regulation of cyclic body processes.

noun. An indoleamine neurotransmitter, 5-hydroxytryptamine, that is involved in depression, appetite, etc., and is crucial in maintaining a sense of well-being, security, etc.

noun. a neurotransmitter involved in e.g. sleep and depression and memory

"Anxiety and depression occur when too much serotonin is reuptaken and spends its time hiding in the nerve cells."