chemical weapon

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

• noun A chemical agent or toxin, such as mustard gas, lewisite, or sarin, that has been prepared for release on the battlefield or within a civilian population in sufficient concentration to cause widespread illness or death.

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

• noun A noxious substance contained in some form of delivery system such as a shell or missile.

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

• noun chemical substances that can be delivered using munitions and dispersal devices to cause death or severe harm to people and animals and plants