Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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