What is conjugate acid base pair?

The acid base pair which are differentiated by one proton (${\mathrm{H}}^{\oplus }$) is called “conjugate acid –base” pair.
Here conjugate acid means – one proton extra
Conjugate base – one proton less
Eg : When HCl is dissolved in water.
conjugate acid of H₂O is $\to$H₃O⁺
$\mathrm{HCl}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \stackrel{{ }^{⊝}}{\mathrm{Cl}}+{\mathrm{H}}_3{\mathrm{O}}^{\oplus }$
conjugate base of HCl is $\to$${\mathrm{Cl}}^{\odot }$