The dollar is the official currency of five U.S. territories, along with 11 foreign countries, territories, and municipalities around the world. It's also the quasi-official currency of several other nations that, in addition to their local currency, commonly accept U.S. dollars.
In addition to being the world's most commonly used currency, the dollar is also the world's reserve currency. This means it's held in large quantities by central banks around the globe.1 Sometimes known as the greenback, the U.S. dollar is also one of the most commonly traded currencies on the foreign exchange market.