/ Dictionary / Index M midline: Meaning and Definition of Find definitions for: mid•line Pronunciation: (mid'līn"), [key] — n. Zool. the median plane of the body of an animal. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: midline (Thesaurus) midlife crisis Midlothian Related Content Daily Word Quiz: feline Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language