/ Dictionary / Index M marsh gas: Meaning and Definition of Find definitions for: marsh' gas" Pronunciation: [key] a gaseous decomposition product of organic matter, consisting primarily of methane. Also called Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. Marshfield marsh grass Related Content Daily Word Quiz: crutiny Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language