/ Dictionary / Index M mean square deviation: Meaning and Definition of Find definitions for: mean' square' devia"tion Pronunciation: [key] variance (def. 3). Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. mean square means test Related Content Daily Word Quiz: maritime Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language