IUPAC-Naming of Dioxin
In the molecule 2,3,7,8-tetrachlorodibenzo-p-dioxin, the "p" means para because each chlorine is para to an oxygen?


No. The p (for “para”) indicates the dioxine isomer.


  In IUPAC nomenclature, shouldn’t “para” be replaced by a number?


Yes. The preferred IUPAC name for this isomer is 1,4-dioxine.

Therefore, the corresponding name of the parent structure of the compound that is given in the question is 2,3,7,8-tetrachlorodibenzo[b,e][1,4]dioxine. The complete name is 2,3,7,8-tetrachlorodibenzo[b,e][1,4]dioxine.

The preferred IUPAC name, however, is 2,3,7,8-tetrachlorooxanthrene.