The U.S. dollar was first designated as the world's currency in the 1944 Bretton Woods Agreement, and it is the most powerful currency in the world. It's backed by the world's largest economy, the United States. The strength of the U.S. economy supports the dollar's use as a global currency.
The term "U.S. dollar" refers to a specific denomination and the U.S. currency in general. It was initially traded as a coin worth its weight in silver or gold and then exchanged as a paper note redeemable in gold. During the 1970s, the gold standard was dropped, and the dollar's value was allowed to float. Today, although its value fluctuates, it's in strong demand.