The Collaborative International Dictionary of English v.0.48:

Protoxide \Pro*tox"ide\, n. [Proto- + oxide: cf. F. protoxide.]
   (Chem.)
   That one of a series of oxides having the lowest proportion
   of oxygen. See Proto-, 2
   (b) .
       [1913 Webster]

   protoxide of nitrogen, laughing gas, now called
      hyponitrous oxide. See under Laughing.
      [1913 Webster]