Definition of ph

The negative logarithm (base 10) of the hydrogen-ion (proton) activity. It denotes the degree of acidity or of basicity of a solution. At the neutral point, the proton activity is equal to the hydroxide ion activity (in chemically pure water, at a temperature of 298.16 K and a pressure of 1.013 bar, the neutral point is at pH 7.0). Acidity increases with decreasing values below the neutral point, and basicity increases with increasing values above the neutral point.

Note that the neutral point is defined as the pH at which the proton activity is equal to the hydroxide ion activity (and NOT as pH 7). At elevated temperature and/or pressure, the pH of water, even when chemically pure, can considerably deviate from 7.0, but this does not mean that it stops to be neutral.

Ref: ASM, 1