From Halopedia, the Halo wiki
Phosphorus is the chemical element that has the symbol P and atomic number 15. The name comes from the Greek: phôs (meaning "light") and phoros (meaning "bearer"). A multivalent nonmetal of the nitrogen group, phosphorus is commonly found in inorganic phosphate rocks.
Due to its high reactivity, Phosphorus is never found as a free element in nature on Earth. One form of Phosphorus (white phosphorus) emits a faint glow upon exposure to oxygen (hence its Greek derivation and the Latin 'light-bearer', meaning the planet Venus as Hesperus or "Morning Star").
Phosphorus is a component of DNA and RNA and an essential element for all living cells. One form it is found in is in ATP, Adenosine Triphosphate, an energy bearing molecule produced by mitochondria. The most important commercial use of Phosphorus-based chemicals is the production of fertilizers.