Wikipedia
A superdollar (also known as a superbill or supernote) is a very high quality counterfeit United States one hundred-dollar bill, alleged by the U.S. Government to have been made by unknown organizations or governments. In 2011 government sources stated that these "counterfeit bills were in worldwide circulation from the late 1980s until at least July 2000" in an extradition court case.
Various groups have been suspected of creating such notes, and international opinion on the origin of the notes varies. The U.S. Government believes that most likely most of these notes were produced in North Korea. Over $35 million of $100 bills were produced by British criminals arrested in 2002. Other sources claimed include criminal gangs in Iran, Russia, China or Syria. The name derives from the fact that the quality of the notes exceeds that of the originals.
More $100 bills circulate in Asia, where U.S. currency is often seen as a safer investment than local currency, and many seizures of supernotes are associated with persons from Korea, Russia, and the Middle East.
A new $100 bill design intended to thwart counterfeiting, incorporating a "3D security ribbon," colorshifting numerals and drawings, and microprinting, entered circulation in 2013.
The term superdollar is sometimes used to refer to a period of extreme strength of the United States dollar, relative to other currencies, particularly in the 1980s. This period ended when the G7 countries, concerned about the American trade deficit and the resulting protectionism, agreed to cooperate in the devaluation of the dollar in the Plaza Accord.