Acidity may be defined as the quality, state or degree of being acid ; or the level of acid in a substance.

Acidity is measured in pH, or the negative log of the activity of the hydrogen ion in an aqueous solution. Solutions with a pH less than 7 are said to be acidic. The pH of an aqueous substance can be measured using a glass electrode and a pH meter, or with the use of pH indicators .

Or a more chemical definition: Acidity is a measure of the number of free hydrogen ions (H+) in a solution that can chemically react with other substances.