/ Dictionary / Index I isidium: Meaning and Definition of Find definitions for: i•sid•i•um Pronunciation: (ī-sid'ē-um), [key] — pl. i•sid•i•a a coralloid outgrowth from the thallus in certain lichens. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. isidioid Isidor Related Content Daily Word Quiz: fidelity Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language