The Collaborative International Dictionary of English v.0.48:

Alumina \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.
   [1913 Webster]

   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.
         [1913 Webster]

WordNet (r) 3.0 (2006):

alumina
    n 1: any of various forms of aluminum oxide occurring naturally
         as corundum [syn: alumina, aluminum oxide, aluminium
         oxide]