hyponitrous oxide

from 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]