/ Dictionary / Index P prostomium: Meaning and Definition of Find definitions for: pro•sto•mi•um Pronunciation: (prō-stō'mē-um), [key] — pl. -mi•a the unsegmented, preoral portion of the head of certain lower invertebrates. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. prostomiate prostoon Related Content Daily Word Quiz: careening Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language