Sentence:  NOAA <WSD>Nitrox</WSD> I is a standard breathing gas mixture of 32% oxygen (±1%); the balance of the gas (68%) is nitrogen. [...] High-pressure storage cylinders, scuba tanks, regulators, and all high-pressure gas transfer equipment that is used with pure oxygen or with <WSD>nitrox</WSD> mixtures that contain more than 40 percent oxygen must be cleaned and maintained for oxygen service. 
Meaning of the word marked with <WSD> tags:  A mixture of nitrogen and oxygen, the nitrogen content being lower than what is normally present in air, which is used in place of air as a breathing gas.