/ Dictionary / Index R rest energy: Meaning and Definition of Find definitions for: rest' en"ergy Pronunciation: [key] — Physics. Physics. the energy equivalent to the mass of a particle at rest in an inertial frame of reference, equal to the rest mass times the square of the speed of light. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. rest cure restful Related Content Daily Word Quiz: vertigo Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language