phos·pho·rus (fŏsˈfər-əs)

1. A highly reactive poisonous nonmetallic element occurring naturally in phosphates, such as hydroxyapatite, and an essential constituent of protoplasm, nerve tissue, and bone. Its radioisotope is used to localize and treat cancers and peritoneal or pleural effusions caused by metastatic disease, to determine blood volume, to study peripheral vascular disease, and to treat blood diseases such as polycythemia vera, chronic myelocytic leukemia, and chronic lymphocytic leukemia. Atomic number 15.
2. A phosphorescent substance.