The solidarity surcharge (Solidaritätszuschlag, aka “Soli”) is a supplementary tax in Germany, which is levied on income tax, corporate income tax, and other taxes and is paid to the federal government. Its introduction in 1991 was due primarily to the cost of the German reunification and the “Reconstruction of East Germany.” The rate of this tax has been 5.5% since 1998 and it is levied throughout the Federal Republic of Germany. Its legal basis is the Solidarity Surcharge Act (SolzG).

Source: Wikipedia, edited (only available in German)