What is "P1" or "P" in…........
 

hi, I want to know about the meaning of term “P1” or "P" in P1 radiation model. Is P stand for pressure? if anybody know please share for knowledge.
Thanks

P-1 is the simplest of the P-N approximations (taking only the lowest order terms) with P-2 and P-3 being higher order approximations to the radiation intensity.

The P is mostly notational and has a confusing history. It could refer to the spherical harmonics or it could also be related to notation commonly used for the Legendre polynomials. Which came first? I don't know. But the P does not have any important meaning, it's historical/convention.