/ Dictionary / Index H hydantoin: Meaning and Definition of Find definitions for: hy•dan•to•in Pronunciation: (hī-dan'tō-in), [key] — n. Pharm. a colorless, needlelike, crystalline compound, CHNO, used in the synthesis of pharmaceutical substances and resins. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: hydantoin (Thesaurus) hyd. hydathode Related Content Daily Word Quiz: inutiae Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language