In Nazi Germany, "euthanasia" referred to the systematic killing of those deemed "unworthy of life" because of genetic diseases or defects. Beginning in fall 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.
Sources: U.S. Holocaust Memorial Museum