/ Dictionary / Index S serotonin: Meaning and Definition of Find definitions for: ser•o•to•nin Pronunciation: (ser"u-tō'nin, sēr"-), [key] — n. Biochem. a neurotransmitter, derived from tryptophan, that is involved in sleep, depression, memory, and other neurological processes. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: serotonin (Thesaurus) serotonergic serotype Related Content Daily Word Quiz: sinister Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language