A conjugate acid, within the Brønsted–Lowry acid–base theory, is a species formed by the reception of a proton (H⁺), by a base—in other words, the base with a hydrogen ion added to it. On the other hand, a conjugate base is merely what is left after an acid has donated a proton in a chemical reaction. Hence, a conjugate base is a species formed by the removal of a proton from an acid. In summary, this can be represented as the following chemical reaction:

Acid + Base Conjugate Base + Conjugate Acid

Johannes Nicolaus Brønsted and Martin Lowry introduced the Brønsted–Lowry theory, which proposed that any compound that can transfer a proton to any other compound is an acid, and the compound that accepts the proton is a base. A proton is a nuclear particle with a unit positive electrical charge; it is represented by the symbol H⁺ because it constitutes the nucleus of a hydrogen atom, that is, a hydrogen cation.