In Nazi usage, "euthanasia" referred to the systematic killing of those Germans whom the Nazis deemed "unworthy of life" because of alleged genetic diseases or defects. Beginning in the fall of 1939, gassing installations were established at Bernburg, Brandenburg, Grafeneck, Hadamar, Hartheim, and Sonnenstein. Patients were selected by doctors and transferred from clinics to one of these centralized gassing installations and killed. After public outrage forced an end to centralized killings, doctors instead administered lethal injections to those selected for "euthanasia" in clinics and hospitals throughout Germany. In this way, the "euthanasia" program continued and expanded until the end of the war.
Source: U.S. Holocaust Memorial Museum