An acid is a substance that donates a proton (H+) to another substance. A base is a substance that accepts a proton from another substance. The conjugate acid of a base is the acid that is formed when the base accepts a proton. The conjugate base of an acid is the base that is formed when the acid donates a proton.

In the case of H2O, the conjugate acid is H3O+. This is because when H2O accepts a proton, it becomes H3O+.