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]

The Elements (07Nov00):

rubidium
Symbol: Rb
Atomic number: 37
Atomic weight: 85.47
Soft silvery metallic element, belongs to group 1 of the periodic table.
Rb-97, the naturally occurring isotope, is radioactive. It is highly
reactive, with properties similar to other elements in group 1, like
igniting spontaneously in air. Discovered spectroscopically in 1861 by
W.
Bunsen and G.R. Kirchoff.