/ Dictionary / Index M M-day: Meaning and Definition of Find definitions for: M-day Pronunciation: (em'dā"), [key] — n. Mil. mobilization day: a day assumed by the Department of Defense as the first day of mobilization, used by the military for planning purposes. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. MDAP M.Des. Related Content Daily Word Quiz: bungled Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language