The Arrhenius definition of an acid is a compound that increases the number of positively charged hydrogen ions, H+, when added to water. According to Arrhenius, a base is any substance that results in the increase of negatively charged hydroxide ions, OH-, when combined with water.
These definitions were developed in 1884 by Svante Arrhenius, a chemist from Sweden.
The Arrhenius definition of acids and bases is one of two regularly accepted definitions. The other is the Bronsted-Lowry definition, which describes an acid as any compound that gives off protons in an aqueous solution and a base as a compound that accepts additional protons.