An anglicization of the ‘thaler’, a silver coin of Germanic origin once widely used in European countries, which has become the unit of currency of the U.S.A. Other countries have also adopted the dollar. but in international currency usage the unit is then usually prefixed with the name of the country to which it applies, e.g. Canadian dollar.
Due to the economic strength of the United States the dollar has long been seen as an attractive currency to hold, and for many years after the Second World War it was almost as acceptable as gold. The U.K. reserves were referred to as gold and dollar reserves. With the growth of other hard currencies such as the deutschmark and consequently the euro the dollar has lost its absolute domination in the currency market, but it is still much sought after, because of its general acceptability in world trade. Many international commodities are officially quoted in U.S. dollars, e.g. gold, oil, etc.
Reference: The Penguin Business Dictionary, 3rd edt.