nitric oxide

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

• noun A colorless, poisonous gas, NO, produced by the partial oxidation of atmospheric nitrogen, by cellular metabolism, and as an intermediate during the manufacture of nitric acid from ammonia. In the body, nitric oxide is involved in oxygen transport to the tissues, the transmission of nerve impulses, and other physiological activities.

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

• noun chemistry the binary compound of nitrogen and oxygen NO; an unstable compound which rapidly oxidizes in the presence of air

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

• noun a poisonous red-brown gas (NO)