/ Dictionary / Index M Mare Nubium: Meaning and Definition of Find definitions for: Ma•re Nu•bi•um Pronunciation: (mär'ā n'bē-um, ny'-, mâr'ē), [key] (Sea of Clouds) a dark plain in the third quadrant of the face of the moon: about 95,000 sq. mi. (245,000 sq. km). Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. mare nostrum* mareograph Related Content Daily Word Quiz: impetuous Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language