There are basically two ways the exchange rate of a country's currency with other currencies is determined (a) A fixed rate - a reduction in this is called a Devaluation and an increase a Revaluation. (b) A 'floating' rate, determined by market forces. A fall under this system is termed a depreciation and a rise an appreciation.
The rate of exchange between two currencies.
The rate, or price, at which one country's currency is exchanged for the currency of another country.