oxygen, colourless, odourless, tasteless gas that makes up nearly 21% of the atmosphere. It is essential to all animal life forms and for many other living systems. Unlike nitrogen, which makes up a much higher proportion of air and is inert, oxygen is highly reactive, being essential for aerobic respiration in plants, animals, and soils. Oxygen interacts with grape juice, must, and wine in good ways (see aeration) and bad ways (see oxidation).
