/ Dictionary / Index R root mean square: Meaning and Definition of Find definitions for: root' mean' square' Pronunciation: [key] — Math. Math. the square root of the arithmetic mean of the squares of the numbers in a given set of numbers. Abbr.: rms Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. rootlet root of unity Related Content Daily Word Quiz: censure Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language