/ Dictionary / Index O optical illusion: Meaning and Definition of Find definitions for: op'tical illu'sion. Pronunciation: [key] See under(def. 4). Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. optical glass optical isomer Related Content Daily Word Quiz: eccentric Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language