I know this is not what P is but on the periodic table of the elements, P is the chemical symbol for Phosphorus.
The first form of elemental phosphorus that was produced (white phosphorus, in 1669) emits a faint glow when exposed to oxygen – hence the name, taken from Greek mythology, Φωσφόρος meaning "light-bearer" (Latin Lucifer), referring to the "Morning Star", the planet Venus (or Mercury).
Funny how that works out.