n. Chem. a colourless tasteless odourless gaseous element, occurring naturally in air, water, and most minerals and organic substances, and essential to plant and animal life. °Symb.: O. oxygen mask a mask placed over the nose and mouth to supply oxygen for breathing. oxygen tent a tentlike enclosure supplying a patient with air rich in oxygen. oxygenous adj.
[ F oxygène acidifying principle (as OXY- 2): it was at first held to be the essential principle in the formation of acids ]