"A guarded compound for the detention or imprisonment of aliens, members of ethnic minorities, political opponents, etc., especially any of the camps established by the Nazis prior to and during World War II for the confinement and persecution of prisoners." www.dictionary.com/browse/concentration-camp?s=ts
Beginning in 1941, the Germans started transports from the ghettos in Poland to concentration camps. They started with people saw as the least useful, sick, old, weak, and very young. The first gassings started in the camp of Belzec. Five more mass killing centers were built at camps in occupied Poland, not only Germany with Chelmno, Sobibor, Treblinka, Majdanek and the most well known, Auschwitz-Birkenau. Jews were deported to the camps from all over Europe. The most people transported happened during 1942. 300,000 people were deported from the Warsaw ghetto.