A measure of how acidic or basic a substance or solution is. pH is measured on a scale of 0 to 14. On this scale, a pH value of 7 is neutral, which means it is neither acidic nor basic. A pH value of less than 7 means it is more acidic, and a pH value of more than 7 means it is more basic. In medicine, having the right pH in the blood and other body fluids is important for the body to work the way it should.

Source: NCI Dictionary of Cancer Terms