WordNet (r) 3.0 (2006):

pH scale
    n 1: (from potential of Hydrogen) the logarithm of the
         reciprocal of hydrogen-ion concentration in gram atoms per
         liter; provides a measure on a scale from 0 to 14 of the
         acidity or alkalinity of a solution (where 7 is neutral and
         greater than 7 is more basic and less than 7 is more
         acidic); [syn: pH, pH scale]