pH is a quantitative measure of the acidity of a solution. pH is represented on a logarithmic scale from zero to fourteen. A solution is considered acidic if its pH is lower than seven, basic if greater than seven and neutral if exactly seven.
For example, pure distilled water has a pH of 7.
More specifically, pH is a measure of the concentration of Hydrogen ions in solution. It is given by the negative of the logarithm of the Hydrogen ion concentration in moles per litre to base 10.