/ Dictionary / Index P protoplanet: Meaning and Definition of Find definitions for: pro•to•plan•et Pronunciation: (prō'tō-plan"it), [key] — n. Astron. the collection of matter, in the process of condensation, from which a planet is formed. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. protophloem protoplasm Related Content Daily Word Quiz: rustic Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language