The Holocaust also referred to as the Shoah was a genocide in which some six million European Jews were killed by Adolf Hitler's Nazi Germany, and the World War II collaborators with the Nazis. The victims included 1.5 million children, and constituted about two-thirds of the nine million Jews who had resided in Continental Europe. A broader definition of the Holocaust includes non-Jewish victims of the Nazi campaign of mass murder.The Romani and the Aktion T4 patients who were mentally and physically disabled are most commonly included among the broader victims, but can also include Soviet prisoners-of war, homosexuals, Jehovah's Witnesses, Slavs - including Soviet citizens, Poles, and other Slavic ethnic groups, blacks, political opponents of the Nazis, and other smaller groups.