The Brønsted acid BH+ formed on protonation of a base B is called the conjugate acid of B, and B is the conjugate base of BH+. (The conjugate acid always carries one unit of positive charge more than the base, but the absolute charges of the species are immaterial to the definition.) For example: the Brønsted acid HCl and its conjugate base Cl- constitute a conjugate acid-base pair.
In Bronsted-Lowry terminology, a reactant and product that differ by a proton, H+.
