Source : Webster's Revised Unabridged Dictionary (1913)

Rubidium \Ru*bid"i*um\, n. [NL., fr. L. rubidus red, fr. rubere
   to be red. So called from two dark red spectroscopic lines by
   means of which it was discovered in the lepidolite from
   Rozena, Moravia. See {Rubicund}.] (Chem.)
   A rare metallic element. It occurs quite widely, but in small
   quantities, and always combined. It is isolated as a soft
   yellowish white metal, analogous to potassium in most of its
   properties. Symbol Rb. Atomic weight, 85.2.

Source : WordNet®

rubidium
     n : a soft silvery metallic element of the alkali metal group;
         burns in air and reacts violently in water; occurs in
         carnallite and lepidolite and pollucite [syn: {Rb}, {atomic
         number 37}]