Usually, a function f:{0,1}n-{0,1}, that takes an n-bit string as input and produces a bit as output.
In mathematics, a finitary boolean function is a function of the form f : Bk → B, where B = {0, 1} is a boolean domain and where k is a nonnegative integer. In the case where k = 0, the "function" is simply a constant element of B.