Devaluation is a reduction in the value of a currency with respect to those goods, services or other monetary units with which that currency can be exchanged.
In common modern usage, it specifically implies an official lowering of the value of a country's currency within a fixed exchange rate system, by which the monetary authority formally sets a new fixed rate with respect to a foreign reference currency. In contrast, depreciation is used for the unofficial decrease in the exchange rate in a floating exchange rate system. The opposite of devaluation is called revaluation.
Depreciation and devaluation are sometimes incorrectly used interchangeably, but they always refer to values in terms of other currencies. Inflation, on the other hand, refers to the value of the currency in goods and services (related to its purchasing power).