The Collaborative International Dictionary of English v.0.48:

Barium \Ba"ri*um\, n. [NL., fr. Gr. bary`s heavy.] (Chem.)
   One of the elements, belonging to the alkaline earth group; a
   metal having a silver-white color, and melting at a very high
   temperature. It is difficult to obtain the pure metal, from
   the facility with which it becomes oxidized in the air.
   Atomic weight, 137. Symbol, Ba. Its oxide called baryta.
   [Rarely written barytum.]
   [1913 Webster]

   Note: Some of the compounds of this element are remarkable
         for their high specific gravity, as the sulphate,
         called heavy spar, and the like. The oxide was called
         barote, by Guyton de Morveau, which name was changed by
         Lavoisier to baryta, whence the name of the metal.
         [1913 Webster]

WordNet (r) 3.0 (2006):

barium
    n 1: a soft silvery metallic element of the alkali earth group;
         found in barite [syn: barium, Ba, atomic number 56]

The Elements (07Nov00):

barium
Symbol: Ba
Atomic number: 56
Atomic weight: 137.34
Silvery-white reactive element, belonging to group 2 of the periodic
table. Soluble barium compounds are extremely poisonous. Identified in
1774 by Karl Scheele and extracted in 1808 by Humphry Davy.