/ Dictionary / Index P protoactinium: Meaning and Definition of Find definitions for: pro•to•ac•tin•i•um Pronunciation: (prō"tō-ak-tin'ē-um), [key] — n. Chem. protactinium. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: protoactinium (Thesaurus) proto- Proto-Algonquian Related Content Daily Word Quiz: feline Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language