phosphorus

noun

1. Archaic any phosphorescent substance or object
2. a nonmetallic chemical element, normally a white, phosphorescent, waxy solid, becoming yellow when exposed to light: it is poisonous and unites easily with oxygen, so that it ignites spontaneously at room temperature: when heated in sealed tubes it is converted into a red form, which is nonpoisonous and less flammable than the white: when heated under a pressure of 10,000 atmospheres it is converted into a black powder: symbol, P; at. no., 15: a radioactive isotope () is used in the diagnosis and treatment of certain diseases, as a tracer in chemical and biochemical research, etc.