Nitric oxide is a colorless gas with the formula NO. It is one of the principal oxides of nitrogen. Nitric oxide is a free radical, i.e., it has an unpaired electron, which is sometimes denoted by a dot in its chemical formula (·N=O or ·NO). An important intermediate in chemical industry, nitric oxide forms in combustion systems and can be generated by lightning in thunderstorms. In mammals, including humans, nitric oxide is a signaling molecule in many physiological and pathological processes. Nitric oxide should not be confused with nitrous oxide (N2O), an anesthetic, or with nitrogen dioxide (NO2), a brown gas and major air pollutant.