/ Dictionary / Index T third law of motion: Meaning and Definition of Find definitions for: third' law' of mo'tion Pronunciation: [key] — Physics. Physics. See under Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. Third International third law of thermodynamics Related Content Daily Word Quiz: procrastinate Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language