Nazi Germany used six extermination camps (Vernichtungslager), also called death camps (Todeslager), or killing centers (Tötungszentren), in Central Europe during World War II to systematically murder over 2.7 million people – mostly Jews – in the Holocaust. The victims of death camps were primarily murdered by gassing, either in permanent installations constructed for this specific purpose, or by means of gas vans. The six extermination camps were Chełmno, Belzec, Sobibor, Treblinka, Majdanek and Auschwitz-Birkenau. Extermination through labour was also used at the Auschwitz and Majdanek death camps. Millions were also murdered in concentration camps, in the Aktion T4 or murdered directly on side.
The idea of mass extermination with the use of stationary facilities, to which the victims were taken by train, was the result of earlier Nazi experimentation with chemically manufactured poison gas during the secretive Aktion T4 euthanasia programme