The term pH has been in use for nearly a century. It is a logarithmic measure of the hydrogen ion concentration ([H+]): pH = -log10[H+]. (Technically, there aren't bare protons (H+) floating around in solutions, but that's another post!) The original symbol was pH. and introduced by Sörensen in 1909. Theories vary as to the origin of the p - all agree it means power but whether in Latin, French or German, seems in dispute. I would hazard it was either German or Latin as the original paper was published in German (Biochem. Zeitschr.). If I were not on the road, I'd look it up.
The modern form pH was introduced in 1920, "as a matter of typographical convenience".
22 minutes ago in The Phytophactor