 Property (τ) is an analogue in discrete group theory of Kazhdan's property (T), and was introduced by Alexander Lubotzky.For a given family of normal subgroups "N" of finite index in Γ, one equivalent formulation is that the Cayley graphs of the groups Γ/"N", all with respect to a fixed symmetric set of generators "S", form an expander family.Therefore superstrong approximation is a formulation of property (τ), where the subgroups "N" are the kernels of reduction modulo large enough primes "p".