It is a calculated adjustment to a country's official exchange rate with respect to a chosen baseline. The baseline can be anything ranging from the wage rates of the country to the price of gold to a foreign currency.

In a fixed exchange rate, only the decision by a country's central bank can change the official value of the currency.

If a government has set 10 units of its currency to 1 USD to revalue, they might change the rate to 5 units per dollar.

In cases where the revaluation shows losses there is a change for those that are trading with margin account for that currency pair to being called for maintenance where they have to deposit the difference in value to meet up with the maintenance margin requirement.