The European single currency introduced in 1999 was the euro.
The euro was launched on 1 January 1999 as a non-physical accounting and electronic currency. At first, participating countries continued using their national notes and coins in daily life, but their exchange rates were permanently fixed against one another and financial transactions were denominated in euros.
The currency replaced the European Currency Unit, or ECU, at a one-to-one rate. Its name had been officially adopted in Madrid in December 1995, after European governments developed plans for economic and monetary union under the Maastricht framework.
Euro banknotes and coins did not enter general circulation until 1 January 2002. That distinction explains a frequent quiz trap: 1999 marks the euro's financial introduction, while 2002 marks its physical cash launch. The euro initially began with 11 participating countries; Greece joined the first cash changeover group later, making 12 countries in 2002.