/ Dictionary / Index C cystine: Meaning and Definition of Find definitions for: cys•tine Pronunciation: (sis'tēn, -tin), [key] — n. Biochem. a crystalline amino acid, CHONS, occurring in most proteins, esp. the keratins in hair, wool, and horn, and yielding cysteine on reduction. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: cystine (Thesaurus) cystidium cystinuria Related Content Daily Word Quiz: tenacious Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language