/ Dictionary / Index S substance P: Meaning and Definition of Find definitions for: substance P Pronunciation: [key] a small peptide released upon stimulation in the nervous system and involved in regulation of the pain threshold. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. substance abuse substandard Related Content Daily Word Quiz: procrastinate Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language