Poison gas is a gas that is poisonous and is usually used to kill people in war or to execute criminals.
poison gas in British English
noun
a gaseous substance, such as chlorine, phosgene, or lewisite, used in warfare to kill or harm
poison gas in American English
any toxic chemical agent in the form of a gas or vapor-forming liquid or solid, esp. one used in chemical warfare to kill or harass through inhalation or contact