The species that remains when a proton (H+) is removed from an acid, H–A. The conjugate base of H2O is HO(-). The stability of the conjugate base is the major factor determining the acidity of a molecule. About 60 orders of magnitude separate the acidity of the strongest acids (e.g. H-I) from the weakest (e.g. H-CH3) which reflects the much greater stability of I(-) compared to CH3(-).