Definition of nitrogen trioxide in US English:

nitrogen trioxide

noun

  • 1The anhydride of nitrous acid, N₂O₃, which is a blue liquid (made greenish by dissolved nitrogen dioxide) which decomposes above 3.5°C (also called dinitrogen trioxide).

  • 2An unstable gaseous oxide of nitrogen, NO₃, identified chiefly by its absorption spectrum.

Origin

Mid 19th century.