\A*lu"mi*na\, n. [L. alumen, aluminis. See {Alum}.]
(Chem.)
One of the earths, consisting of two parts of aluminium and
three of oxygen, {Al2O3}.
Note: It is the oxide of the metal aluminium, the base of
      aluminous salts, a constituent of a large part of the
      earthy siliceous minerals, as the feldspars, micas,
      scapolites, etc., and the characterizing ingredient of
      common clay, in which it exists as an impure silicate
      with water, resulting from the decomposition of other
      aluminous minerals. In its natural state, it is the
      mineral corundum.