The meaning of the “p” in “pH” is unknown. ~ Wikipedia
pH is a measure of how base or acidic an aqueous solution is. In a chemical reaction, an acid is a molecule or ion capable of donating a cation of protium (1H+); a base is accepting of 1H+.
(The term proton is sometimes used for 1H+. This usage comes from the 1923 concept of acids and bases independently developed by chemists Johannes Brønsted and Martin Lowry. Of course, using proton here is inexact. The term hydron is also sometimes used, though generally disfavored, as hydron is the general name for H+, which is the cationic form of hydrogen, regardless of isotope. Hence, hydron represents the nucleus of 1H+, 2H+ (D+) (deuterium isotope), and 3H+ (T+) (tritium isotope). It may seem strange that chemistry has no exact term for 1H+. There have been numerous characterizations of acids and bases, including a 1923 electronic theory of acid-base reactions by Gilbert Lewis. The Brønsted–Lowry conceptualization is a theory for which exceptions are known. Much of the intricacy of chemistry remains to be discovered.)
Water is neutral (pH = 7). Acids have a pH < 7, while bases have a >7 pH.
pH was coined in 1924. The “H” in pH stands for hydrogen, but the meaning of “p” is a long-standing mystery. p = power is a common guess. pH: the power of hydrogen.
While acidity is related to the concentration of hydrogen ions, it is not ionic concentration per se that confers pH, but instead the activity factor of a solution, which is the tendency of hydrogen ions to interact with other elements in a solution. Acid-base reactions are tangled enterprises.