Currency devaluation and overvaluation refer to adjustments in a country's official exchange rate under a fixed exchange rate system. Devaluation occurs when a country officially lowers the value of its currency, while overvaluation is when a country raises the value of its currency. In contrast, under a floating exchange rate system, a currency's value fluctuates based on market forces rather than government actions. Depreciation describes a decrease in value due to market forces, while appreciation refers to an increase in value due to market forces.