pH is a measure of the acidity or alkalinity (basic-ness) of a solution. Pure or neutral water has a pH of 7.0 Acids are defined as those solutions that have a pH less than 7, while bases are defined as those solutions that have a pH greater than 7.
The pH scale is logarithmic. Unlike linear scales, which have a constant relationship between the item being measured and the value reported, each individual pH unit is a factor of 10 different than the next higher or lower unit. For example, a change in pH from 2 to 3 represents a 10-fold decrease in acidity, and a shift from 2 to 4 respresents a 100-fold (10 x 10) decrease in acidity.
More specifically, pH is approximately equal to the negative logarithm of hydrogen ion concentration expressed in molarity. In other words, pH is a measure of the hydrogen ion concentration of a solution.
Parts of this definition were adapted from Biology-Online.org.