Altbier (often abbreviated to Alt) is a dark, top-fermented type of beer from Düsseldorf and the Niederrhein region in Germany.
The name Altbier, which literally means old beer, refers to the old brewing style (top-fermenting yeast and dark malt). Up to the 1950s, Alt was also called Düssel (from Düsseldorf), but since the term is not a Protected Designation of Origin, Altbier may also be produced outside of the Düsseldorf region. Traditionally, Altbiers are conditioned for a longer than normal periods of time. Alt beers are dark, copper colored, brewed from dark malts and well-hopped.
Summary from www.beertutor.com