conjugate acid–base pair
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.
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1099 (