nerve gas

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

• noun Any of various poisonous compounds, dispersed in vapor or aerosol form, that interfere with the functioning of nerves by inhibiting acetylcholinesterase.

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

• noun A toxic chemical compound and banned weapon of mass destruction which, when inhaled or ingested, impairs nervous system functions, typically causing extreme physical distress and death by asphyxiation.

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

• noun a toxic gas that is inhaled or absorbed through the skin and has harmful effects on the nervous and respiratory system