What is the definition or description of: nitric oxide?

A therapeutic gas. A gas that can directly relax vascular smooth muscle. It is used in the critical care setting to treat pulmonary hypertension. This can be seen in premature neonates with bronchiopulmonary dysplasia or neonates with congenital heart disease. It may be used in adults with severe pulmonary hypertension of various causes.
A molecular compound. This is a molecule which is part of many physiologic processes by acting as a 'signalling molecule' causing vasodilation. It is also the byproduct of combustion and is a component of automobile exhaust.