[ noun ] an oxide containing three atoms of oxygen in the molecule <noun.substance>
Trioxide \Tri*ox"ide\, n. [Pref. tri- + oxide.] (Chem.) An oxide containing three atoms of oxygen; as, sulphur trioxide, {SO3}; -- formerly called {tritoxide}.
EIC Laboratories, a Norwood, Mass., contract-research concern, has fabricated a prototype electrochromic window with tungsten trioxide and other chemicals.
Sulfur trioxide is used in the production of sulfuric acid.