A reserve currency (or anchor currency) is a foreign currency that is held in significant quantities by central banks or other monetary authorities as part of their foreign exchange reserves. The reserve currency can be used in international transactions, international investments and all aspects of the global economy. It is often considered a hard currency or safe-haven currency.
The United Kingdom's pound sterling was the primary reserve currency of much of the world in the 19th century and first half of the 20th century. However, by the middle of the 20th century, the United States dollar had become the world's dominant reserve currency. The world's need for dollars has allowed the United States government to borrow at lower costs, giving the United States an advantage in excess of US$100 billion per year.