Nitrox refers to any gas mixture composed (excluding trace gases) of nitrogen and oxygen; this includes normal air which is approximately 78% nitrogen, 21% oxygen, and 1% other gases, primarily argon. However, in scuba diving, nitrox is normally differentiated and handled differently from air. The most common use of nitrox mixtures containing higher than normal levels of oxygen is in scuba, where the reduced percentage of nitrogen is advantageous in reducing nitrogen uptake in the body's tissues and so extending the possible dive time, and/or reducing the risk of decompression sickness (also known as the bends).
Read more about Nitrox: Purpose, Naming, Richness of Mix, Cylinder Markings To Identify Contents, History, Nitrox in Nature