Purchasing power parity (PPP) refers to a theory that the price of internationally traded commodities should be the same in every country, and hence the exchange rate between the two currencies should be the ratio of prices in the two countries. Relative PPP holds that if the spot exchange rate between two countries starts in equilibrium, any change in the differential rate of inflation between them tends to be offset over the long run by an equal but opposite change in the spot exchange rate.
Purchasing Power Parity is the concept that homogeneous goods cannot have more than one price measured in any one currency. If the price increases domestically, the domestic currency will depreciate so that the price denominated in foreign currency remains the same.