/ Dictionary / Index M middle plane: Meaning and Definition of Find definitions for: mid'dle plane'. Pronunciation: [key] See(def. 1). Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. Middle Persian Middle River Related Content Daily Word Quiz: penchant Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language