phos·pho·rus 
(fŏsfər-əs)
(fŏsfər-əs)n.
1. Symbol P A highly reactive, poisonous, nonmetallic element occurring naturally in phosphates, especially apatite, and existing in three allotropic forms, white (or sometimes yellow), red, and black (or violet). An essential element for biological cells, it is used in safety matches, pyrotechnics, incendiary shells, and fertilizers and to protect metal surfaces from corrosion. Atomic number 15; atomic weight 30.9738; melting point (white) 44.15°C; boiling point 280.5°C; specific gravity (white) 1.82, (red) 2.16, (black) 2.25 to 2.69; valence 3, 4, 5. See Periodic Table.
2. A phosphorescent substance.
3. Phosphorus Astronomy See Lucifer.
[Modern Latin phōsphorus, substance or organism that emits light, phosphorus, Latin Phōsphorus, morning star, from Greek phōsphoros, bringing light, morning star : phōs, light; see bhā-1 in the Appendix of Indo-European roots + -phoros, -phorous.]