Breadcrumb Home > Dictionary > Index S Sinus Iridum: Meaning and Definition of Find definitions for: Si•nus Ir•i•dum Pronunciation: (sē'nus ir'i-dum, sī'-), [key] (Bay of Rainbows) a semicircular dark plain in the second quadrant of the face of the moon. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. sinus sinusitis ✖ Related Content Daily Word Quiz: donnybrook Analogy of the Day: Today’s Analogy Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language