/ Dictionary / Index U ultramarathon: Meaning and Definition of Find definitions for: ul•tra•mar•a•thon Pronunciation: (ul"tru-mar'u-thon", -thun), [key] — n. any footrace of 50 or more miles. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. ultramafic ultramarine 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