/ Dictionary / Index M mail-in: Meaning and Definition of Find definitions for: mail-in Pronunciation: (māl'in"), [key] — adj. conducted or responding by mail: a mail-in referendum. —n. something conducted or returned by mail, as a questionnaire or vote. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. Mailgram mailing Related Content Daily Word Quiz: cautious Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language