Bronsted–Lowry Concept
According to this concept, “An acid is defined as a substance which has the tendency to give a proton (H+) and a base is defined as a substance which has a tendency to accept a proton. In other words, an acid is a proton donor whereas a base is a proton acceptor.”
⇌ 
…..(i)
⇌ 
…..(ii)
(i) 
and 
are acids because they donate a proton to 
.(ii) 
and 
are bases because they accept a proton from water.
In reaction (i), in the reverse process, H3O+ can give a proton and hence is an acid while Cl– can accept the proton and hence is a base. Thus there are two acid-base pairs in reaction (i). These are HCl – Cl– and H3O+–H2O. These acid-base pairs are called conjugate acid-base pairs.
Conjugate acid ⇌ Conjugate base 
Conjugate base of a strong acid is a weak base and vice a versa. Weak acid has a strong conjugate base and vice a versa.