protonation constant

https://doi.org/10.1351/goldbook.P04909
The equilibrium constant KHn for the addition of the nth proton to a charged or uncharged ligand. The cumulative protonation constant, β H n, is the equilibrium constant for the formation of HnL from nH+ and L. The same equilibrium constant may be described in several ways, depending on how the ligand, L, or the acid, has been defined.
Source:
Orange Book, 2nd ed., p. 13 (http://media.iupac.org/publications/analytical_compendium/)