Reserve Currency

What Does Reserve Currency Mean?
A foreign currency held by central banks and other major financial institutions as a means to pay off international debt obligations, or to influence their domestic exchange rate.

Investopedia explains Reserve Currency
Currently, the U.S. dollar is the primary reserve currency used by other countries. A very large percentage of commodities such as gold and oil are usually priced in U.S dollars, causing other countries to hold this currency to pay for these goods. A large debate still continues about whether or not the U.S. dollar will stay the main reserve currency or if it will shift over to the euro.