pH is a measure of the acidity or alkalinity of a substance, and the term "pH" is short for hydrogen potential. By convention, pH is the negative log of the hydrogen potential of a solution or mixture,

where represents the concentration of hydrogen ions in units of moles per liter of volume.

Substances with high hydrogen ion concentration, specifically those with a pH between 0 and 7, are generally considered acidic (although the technical definition of an acidis a bit tricky). Those with a pH between 7 and 14 are considered basic.