What is "gruppen"

Gruppen for three orchestras (1955–57) is amongst the best-known compositions of German composer Karlheinz Stockhausen, and is Work Number 6 in the composer's catalog of works. Gruppen is "a landmark in 20th-century music . . . probably the first work of the post-war generation of composers in which technique and imagination combine on the highest level to produce an undisputable masterpiece" .