Breadcrumb Home > Dictionary > Index M M-line: Meaning and Definition of Find definitions for: M-line Pronunciation: (em'līn"), [key] — n. Physics. one of a series of linesin the x-ray spectrum of an atom corresponding to radiationcaused by the transition of an electron to the M-shell. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. MLG. Mlle. ✖ Related Content Daily Word Quiz: quadrumanous Analogy of the Day: Today’s Analogy Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language