/ Dictionary / Index R rubidium: Meaning and Definition of Find definitions for: ru•bid•i•um Pronunciation: (r-bid'ē-um), [key] — n. Chem. a silver-white, metallic, active element resembling potassium, used in photoelectric cells and radio vacuum tubes. Symbol: Rb; at. wt.: 85.47; at. no.: 37; sp. gr.: 1.53 at 20°C. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: rubidium (Thesaurus) rubicund rubidium-strontium dating Related Content Daily Word Quiz: panacea Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language