/ Dictionary / Index P protactinium: Meaning and Definition of Find definitions for: prot•ac•tin•i•um Pronunciation: (prō"tak-tin'ē-um), [key] — n. Chem. a radioactive, metallic element. Symbol: Pa; at. no.: 91. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: protactinium (Thesaurus) prot- protagonist Related Content Daily Word Quiz: fervor Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language