trioxide

from WordNet (r) 3.0 (2006)

trioxide
    n 1: an oxide containing three atoms of oxygen in the molecule

from The Collaborative International Dictionary of English v.0.48

Trioxide \Tri*ox"ide\, n. [Pref. tri- + oxide.] (Chem.)
   An oxide containing three atoms of oxygen; as, sulphur
   trioxide, {SO3}; -- formerly called {tritoxide}.
   [1913 Webster]