pH is a unit of measurement for the amount of hydrogen ions in a water solution. It is a logarithmic scale, which each full unit indicating an increase of ten times the concentration of the previous unit. Pure water has a pH of 7. A solution with a pH of less than 7 is considered acidic, and a solution with a pH of more than 7 is considered an alkaline.
Different portions of the body have different normal pH levels, and this can be helpful in a diagnosis. Stomach acid can have a pH as low as 1.5. Skin is normally slightly acid as well, with a pH of around 5.5. However, pancreatic secretions are very alkaline, with a pH of about 8.5.pH at Wikipedia