/ Dictionary / Index P prothallium: Meaning and Definition of Find definitions for: pro•thal•li•um Pronunciation: (prō-thal'ē-um), [key] — pl. -thal•li•a the gametophyte of ferns and related plants. the analogous rudimentary gametophyte of seed-bearing plants. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. prothalamium prothallus 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