/ Dictionary / Index P promethium: Meaning and Definition of Find definitions for: pro•me•thi•um Pronunciation: (pru-mē'thē-um), [key] — n. Chem. a rare-earth, metallic, trivalent element. Symbol: Pm; at. no.: 61. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: promethium (Thesaurus) Prometheus Unbound* prominence Related Content Daily Word Quiz: lyceum Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language