/ Dictionary / Index I iodine 131: Meaning and Definition of Find definitions for: iodine 131 Pronunciation: [key] — Chem. Chem. a radioactive isotope of iodine having a mass number 131 and a half-life of 8.6 days, used in the diagnosis and treatment of disorders of the thyroid gland. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. iodine iodism Related Content Daily Word Quiz: charlatan Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language