Source : Webster's Revised Unabridged Dictionary (1913)

Dioxide \Di*ox"ide\ (?; 104), n. [Pref. di- + oxide.] (Chem.)
   (a) An oxide containing two atoms of oxygen in each molecule;
       binoxide.
   (b) An oxide containing but one atom or equivalent of oxygen
       to two of a metal; a suboxide. [Obs.]

   {Carbon dioxide}. See {Carbonic acid}, under {Carbonic}.

Source : WordNet®

dioxide
     n : an oxide containing two atoms of oxygen in the molecule