The name Plaster of Paris is due to the fact that -

The name Plaster of Paris came from the fact that it was first of all made by heating gypsum which was mainly found in Paris. Gypsum is calcium sulphate dihydrate that is calcium sulphate containing two molecules of water of crystallization ${\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}$.