The United States dollar (sign: $; code: USD; also abbreviated US$ and referred to as the dollar, U.S. dollar, US dollar or American dollar) is the official currency of the United States and its overseas territories. It is a Federal Reserve Note and consists of 100 smaller cent units.
The U.S. dollar is fiat money. It is the currency most used in international transactions and is the world's most dominant reserve currency.
Several countries use it as their official currency, and in many others it is the de facto currency. Besides the United States, it is also used as the sole currency in two British Overseas Territories in the Caribbean: the British Virgin Islands and the Turks and Caicos islands. A few countries use only the US Dollar for paper money, while the country mints its own coins, or also accepts US coins that can be use as payment in US dollars. Some cities around the world use the U.S. dollar as the de facto currency alongside their own.
It is worth noting that, when old denominations and coin formats are withdrawn, they are just "de-authorised", i.e. no longer allowed to be produced, but are never officially withdrawn from circulation or have their legal tender status revoked. The Coinage Act of 1965 proclaims all coin and currency of the United States good to any amount for payment of public and private debt; interestingly, this was not the case for some of the old coins while they circulated - 19th century copper and bronze for example was not legal tender, since at that time only precious metal coinage was considered legal tender. | 
