We use the term "reserve currency" when referring to the common use of the dollar by other countries when settling their international trade accounts. For example, if Canada buys goods from China, it may pay China in US dollars rather than Canadian dollars, and vice versa. However, the foundation from which the term originated no longer exists, and today the dollar is called a "reserve currency" because foreign countries hold it in great quantity simply to facilitate trade. The first reserve currency was the British Pound Sterling. Because the Pound was "good as gold", many countries found it more convenient to hold Pounds rather than gold itself during the age of the gold standard. The world's great trading nations settled their trade in gold, but they might hold Pounds rather than gold, with the confidence that the Bank of England would hand over the gold at a fixed exchange rate upon presentment. Toward the end of World War II the US dollar was given this status by treaty following the Bretton Woods Agreement. The International Monetary Fund (IMF) was formed with the express purpose of monitoring the Federal Reserve's commitment to Bretton Woods by ensuring that the Fed did not inflate the dollar and stood ready to exchange dollars for gold at $35 per ounce. Thusly, countries had confidence that their dollars held for trading purposes were as "good as gold" as had been the Pound Sterling at one time.
However, the Fed did not maintain its commitment to the Bretton Woods Agreement and the IMF did not attempt to force it to hold enough gold to honor all its outstanding currency in gold at $35 per ounce. The Fed was called to account in the late 1960s first by France and then by others until its gold reserves were so low that it had no choice by to revalue the dollar at some higher exchange rate or abrogate its responsibilities to honor dollars for gold entirely. To it everlasting shame, the US chose the latter and "went off the gold standard" in September 1971.
Nevertheless, the dollar was still held by the great trading nations, because it still performed the useful function of settling international trading accounts. There was no other currency that could match the dollar, despite the fact that it was "delinked" from gold. There are two characteristics of a currency that make it useful in international trade: one, it is issued by a large trading nation itself, and, two, the currency holds its value vis a vis other commodities over time. These two factors create a demand for holding a currency in reserve. Although the dollar was being inflated by the Fed, thusly losing its value vis a vis other commodities over time, there was no real competition. The German Deutschemark held its value better, but German trade was a fraction of US trade, meaning that holders of marks would find less to buy in Germany than holders of dollars would find in the US. So demand for the mark was lower than demand for the dollar. Of course, psychological factors entered the demand for dollars, too, since the US was the military protector of all the Western nations against the communist countries.
Today we are seeing the beginnings of a change. The Fed has been inflating the dollar massively, reducing its purchasing power vis a vis other commodities, causing many of the world's great trading nations to use other monies upon occasion. I have it on good authority that DuPont settles many of its international accounts in Chinese yuan and European euros. There may be other currencies that are in demand for trade settlement by other international countries. One factor that has helped the dollar retain its reserve currency demand is that the other currencies have been inflated, too. For example, Japan has inflated the yen to a greater extent than the dollar in its foolish attempt to revive its stagnant economy by cheapening its currency. So the monetary destruction disease is not limited to the US alone!
The dollar is very susceptible to losing its vaunted reserve currency position by the first major trading country that stops inflating its currency. There is evidence that China understands what is at stake; it has increased its gold holdings and has instituted controls to prevent gold from leaving China. Should the world's second largest economy and one of the world's greatest trading nations tie its currency to gold, demand for the yuan would increase and demand for the dollar would decrease. In practical terms this means that the world's great trading nations would reduce their holdings of dollars, and dollars held overseas would flow back into the US economy, causing prices to increase. How much would they increase? It is hard to say, but keep in mind that there is an equal amount of dollars held outside the US as inside the US.
Perhaps only such non-coercive pressure form a sovereign country like China can wake up the Fed to the consequences of its actions and force it to end its Quantitative Easing policy.