- An oxide containing three oxygen atoms per molecule.
- An oxide containing three atoms of oxygen; ; -- formerly called <altname>tritoxide</altname>.
- any <xref>oxide</xref> containing three <xref>oxygen</xref> <xref>atoms</xref> in each <xref>molecule</xref>
- any <xref>organic</xref> compound of <xref>general formula</xref> R-OOO-R', derived from <xref>trioxidane</xref>
- an oxide containing three atoms of oxygen in the molecule