What is "phosphorus trioxide"

Phosphorus trioxide is the chemical compound with the molecular formula PO. Although it should properly be named tetraphosphorus hexoxide, the name phosphorus trioxide preceded the knowledge of the compound's molecular structure, and its usage continues today. This colorless solid is structurally related to adamantane. It is formally the anhydride of phosphorous acid, HPO, but cannot be obtained by the dehydration of the acid. It is a white, waxy, crystalline and highly toxic solid.