/ Dictionary / Index P prothallus: Meaning and Definition of Find definitions for: pro•thal•lus Pronunciation: (prō-thal'us), [key] — pl. -thal•li prothallium. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. prothallium prothesis Related Content Daily Word Quiz: cygnet Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language