Pronunciation: (fos'fur-us),[key] —n., —pl.-pho•riPronunciation: (-fu-rī").[key] 1. Chem.a solid, nonmetallic element existing in at least three allotropic forms, one that is yellow, poisonous, flammable, and luminous in the dark, one that is red, less poisonous, and less flammable, and another that is black, insoluble in most solvents, and the least flammable. The element is used in forming smoke screens, its compounds are used in matches and phosphate fertilizers, and it is a necessary constituent of plant and animal life in bones, nerves, and embryos. Symbol: P; at. wt.: 30.974; at. no.: 15; sp. gr.: (yellow) 1.82 at 20°C, (red) 2.20 at 20°C, (black) 2.25–2.69 at 20°C.
2. any phosphorescent substance.