/ Dictionary / Index M mercury arc: Meaning and Definition of Find definitions for: mer'cury arc" Pronunciation: [key] a bluish-green electric arc that has passed through a mercury-vapor cathode. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. mercury mercury barometer Related Content Daily Word Quiz: rustic Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language