/ Dictionary / Index M mean square: Meaning and Definition of Find definitions for: mean' square' Pronunciation: [key] the mean of the squares of a set of numbers. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. meanspirited mean square deviation 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