Source : Webster's Revised Unabridged Dictionary (1913)

Ester \Es"ter\, n. [A word invented by L. Gmelin, a German
   chemist.] (Chem.)
   An ethereal salt, or compound ether, consisting of an organic
   radical united with the residue of any oxygen acid, organic
   or inorganic; thus the natural fats are esters of glycerin
   and the fatty acids, oleic, etc.

Source : WordNet®

ester
     n : formed by reaction between an acid and an alcohol with
         elimination of water