Devaluation is reduction in the value of a currency. Typically it refers to the value of the currency relative to some specific baseline, such as foreign currencies or gold, rather than prevailing wages and prices.
The term inflation is generally used differently from the term currency devaluation. The former applies to the value of the currency within the national region of use, whereas the latter applies to the external value on international markets. The extent to which these two phenomena are related is open to economic debate.
The term is most often used when a currency has a defined value relative to the baseline. Historically, early currencies were typically coins stamped from gold or silver by an issuing authority which certified the weight and purity of the precious metal. A government in need of money and short on precious metal might abruptly lower the weight or purity of the coins without announcing this, or else decree that the new coins had equal value to the old, thus devaluing the currency. This gave rise to Gresham's Law, which stated that "bad money drives out good", i.e., if pure gold coins and false coins are decreed to have equal value, people will use the false coins for currency and hide the good coins or melt them down into gold.
Later, paper currencies were issued, and governments decreed them to be redeemable for gold or silver (a gold standard). Again, a government short on gold or silver might devalue by abruptly decreeing a reduction in the currency's redemption value, reducing the value of everyone's holdings. Naturally, a government which made a habit of doing this would lead its citizens to hold gold or silver in place of the government's notes, so such governments would often outlaw private hoarding of precious metal in order to prevent Gresham's Law from taking effect.
Present day currencies are usually fiat currencies with insignificant inherent value. The value of currency is determined by the interplay of money supply and money demand. As some countries hold floating exchange rates, others maintain fixed exchange rate policy against the United States dollar or other major currencies. These fixed rates are usually maintained by a combination of legally enforced capital controls or through government trading of foreign currency reserves to manipulate the money supply. Under fixed exchange rates, persistant capital outflows or trade deficits may lead countires to lower or abandon their fixed rate policy, resulting in a devaluation (as persistant surpluses and capital inflows may lead them towards revaluation).
In an open market, the perception that a devaluation is imminent may lead speculators to sell the currency in exchange for the country's foreign reserves, increasing pressure on the issuing country to make an actual devaluation. When speculators buy out all of the foreign reserves, a balance of payments crisis occurs. Economists Paul Krugman and Maurice Obsfelled present a theoretical model in which they state that the balance of payments crisis occurs when the real exchange rate (exchange rate adjusted for relative price differences between countries) is equal to the nominal exchange rate (the stated rate)(Krugman, Paul and Maurice Obstfelled. International Economics (2000), Chapter 17 [Appendix II]). In practice, the onset of crisis has typically occurred after the real exchange rate has depreciated below the nominal rate. The reason for this is that speculators do not have perfect information; they sometimes find out that a country is low on forign reserves well after the real exchange rate has fallen. In these circumstances, the currency value will fall very far very rapidily. This is what occurred during the Mexican Peso Crisis.
Generally, a steady process of inflation is not considered a devaluation, although if a currency has a high level of inflation, its value will naturally fall against gold or foreign currencies. Especially where a country deliberately prints money (a usual cause of hyperinflation) to cover a persistent budget deficit without borrowing, this may be considered a devaluation.
In some cases, a country may revalue its currency higher (the opposite of devaluation) in response to positive economic conditions, to lower inflation, or to please investors and trading partners. This would imply that existing currency increased in value, as opposed to the case where a country issues a new currency to replace an old currency that had declined excessively in value. (such as Romania in 2005, Russia in 1997, or Germany in 1923)