Chem. [f. TRI- 5 + OXIDE.] A compound of three atoms of oxygen with an element or radical; e.g., sulphur trioxide, SO3; nitrogen trioxide, N2O3.
1868. Fownes, Elem. Chem. (ed. 10), 134. A series of oxides containing quantities of oxygen in the proportion of the numbers 1, 2, 3, united with a constant quantity of another element, are distinguished as monoxide, dioxide, and trioxide respectively.
1911. Roscoe & Schorlemmer, Treat. Chem. (ed. 4), I. 723. Boron Trioxide, B2O3,… is obtained when boron burns in the air, or in oxygen.
