a colorless gas, NO, prepared by the action of nitric acid on copper or directly from the air by various processes
See nitric oxide in American Heritage Dictionary 4
noun
A colorless, poisonous gas, NO, produced as an intermediate during the manufacture of nitric acid from ammonia or atmospheric nitrogen and as a product of cellular metabolism. In the body, nitric oxide is involved in oxygen transport to the tissues, the transmission of nerve impulses, and other physiological activities.