The Collaborative International Dictionary of English v.0.48:

Peroxide \Per*ox"ide\, n. (Chem.)
   An oxide containing more oxygen than some other oxide of the
   same element. Formerly peroxides were regarded as the highest
   oxides. Cf. Per-, 2.
   [1913 Webster]

WordNet (r) 3.0 (2006):

peroxide
    n 1: a viscous liquid with strong oxidizing properties; a
         powerful bleaching agent; also used (in aqueous solutions)
         as a mild disinfectant and (in strong concentrations) as an
         oxidant in rocket fuels [syn: hydrogen peroxide,
         peroxide]
    2: an inorganic compound containing the divalent ion -O-O-
    v 1: bleach with peroxide; "She must peroxide her hair-it looks
         unnaturally blond"