IPA: //traɪˈɒksaɪd//
KK: /traɪˈɒksaɪd/
A chemical compound that has three oxygen atoms in each molecule.
Sulfur trioxide is used in the production of sulfuric acid.
Trioxide is formed from "tri-" (meaning three) and "oxide" (from the Greek "oxys", meaning acid or sharp, and "-ide", a suffix used in chemistry to denote a binary compound). The word refers to a chemical compound containing three oxygen atoms.
Think of 'tri-' meaning three, and 'oxide' referring to a compound with oxygen. This helps you remember that trioxide is a compound with three oxygen atoms.