phosphorus
//ˈfɒs.fər.əs//
Noun
1
A chemical element with the symbol P and atomic number 15, existing in several allotropic forms including white phosphorus (which glows in the dark and is highly reactive) and red phosphorus.
White phosphorus glows in the dark and must be stored underwater because it is highly reactive.
2
A substance or material containing phosphorus, especially one that exhibits phosphorescence or luminescence.
The watch hands are coated with phosphorus so they glow in the dark.