rubidium(n = noun.substance) atomic number 37, rb - 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;
is a kind of metal, metallic element

   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]

rubidium, n. Chem. a soft silvery element occurring naturally in various minerals and as the radioactive isotope rubidium-87.

Usage:
Symb.: Rb.

Etymology:
L rubidus red (with ref. to its spectral lines)