Devaluation in modern monetary policy 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. "Devaluation" means 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 to describe a decrease in a currency's value (relative to other major currency benchmarks) due to market forces, not government or central bank policy actions. Under the second system central banks maintain the rates up or down by buying or selling foreign currency, usually but not always USD. The opposite of devaluation is called revaluation.