A currency is a system of money in general use in a particular country or economic area, serving as a medium of exchange, unit of account, and store of value. In forex trading, currencies are traded in pairs with their values determined by supply and demand in the global foreign exchange market. Major currencies like the US dollar, euro, and Japanese yen account for the majority of daily forex trading volume.
Example
“The US dollar (USD) is the world's primary reserve currency and is involved in approximately 88% of all forex transactions, making it the most widely traded currency in the global foreign exchange market.”