The Collaborative International Dictionary of English v.0.48:

Sesquioxide \Ses`qui*ox"ide\, n. [Sesqui- + oxide.] (Chem.)
   An oxide containing three atoms of oxygen with two atoms (or
   radicals) of some other substance; thus, alumina, Al2O3 is
   a sesquioxide.
   [1913 Webster] Sesquipedal