method of executing condemned prisoners by lethal gas; first used in Nevada in 1924 in an effort to provide a more humane form of capital punishment; prisoner is strapped in a chair in a sealed chamber in which poisonous fumes, such as cyanide, are released; if the prisoner breathes deeply, death is almost instantaneous and painless; by the second half of the 20th century, 11 U.S. states had adopted the gas chamber as the method of execution; also used by Nazi Germany during the Holocaust to kill inmates of concentration camps.