pH: A measure of the acidity or alkalinity of a fluid. The pH of any fluid is the measure of its hydrogen ion (H+) concentration relative to that of a given standard solution. The pH may range from 0 to 14, where 0 is most acid, 14 most basic, and 7 is neutral.
Mathematically, the pH is the logarithm (to the base 10) of the reciprocal of the hydrogen ion concentration, [H+]. The pH is approximately equal to the negative logarithm of the H+ ion concentration expressed in molarity.