An acidity function is a measure of the acidity of a medium or solvent system, usually expressed in terms of its ability to donate protons to (or accept protons from) a solute (Brønsted acidity). The pH scale is by far the most commonly used acidity function, and is ideal for dilute aqueous solutions. Other acidity functions have been proposed for different environments, most notably the Hammett acidity function, H₀, for superacid media and its modified version H_- for superbasic media. The term acidity function is also used for measurements made on basic systems, and the term basicity function is uncommon.