pH

A chemical measure of acidity. pH is the negative log (base10) of the hydrogen ion concentration, and is expressed as a whole number on a scale ranging from 1 through 14, with 1 being the most acidic, 7 is neutral, and 14 the most basic.

Links
  http://www.miamisci.org/ph/
http://www.osearth.com/resources/recall/acidrain.shtml