It is the ability that a steel has to be quenched. It depends almost exclusively on the chemical composition and is defined in the T.T.T. curve, although by the Jominy test the hardenability can be determined in steels.
It should not be confused with the penetration of quenching: distance from the periphery in which the piece has a completely martensitic structure. It depends on the size of the piece and on the severity of the quenching (cooling agent and quenching medium).