/ Dictionary / Index P planarian: Meaning and Definition of Find definitions for: pla•nar•i•an Pronunciation: (plu-nâr'ē-un), [key] — n. Zool. any of various free-swimming, mostly freshwater flatworms of the class Turbellaria, having an undulating or sluglike motion: popular in laboratory studies for the ability to regenerate lost parts. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: planarian (Thesaurus) planar planate Related Content Daily Word Quiz: inutiae Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language