What is the medical symbol for letter p with a circle around it?

I'm not aware of a circle P having any significance in medicine. It is commonly used as a copyright mark for recording [similar to circle c (C) - copyright, and circle r (R) - registered]. I believe circle p - (P) - is meant to be performance recording copyright or phonograph copyright (or something similar)

Also, (P)i is the symbol for a phosphate group in cellular respiration. The phosphate group is added to ADP to create ATP.

The formula is ADP+ (P)i -> ATP+ H2O