The full name of a photon is simply
"photon". It's not an acronym, so it doesn't have a longer or full form like some other scientific terms. The word "photon" comes from the Greek word
phos, meaning "light." In physics, a photon is a particle of light, or more generally, a quantum of electromagnetic radiation. It has no mass, but it carries energy and momentum, and it travels at the speed of light in a vacuum.