The Collaborative International Dictionary of English v.0.48:

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 of the alkali metal series, atomic
   number 37. 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, 85.48.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rubidium
    n 1: 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: rubidium,
         Rb, atomic number 37]