gas chamber

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An enclosed space where a poisonous gas is emitted to kill the person inside, often as a form of execution.
• noun A similar space used for the anesthetization or killing of an animal.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a sealed chamber in which people are executed with poison gas

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun instrument of execution consisting of a sealed chamber into which poison gas is introduced; used to kill people or animals