The Church numerals are a family of higher-order functionals that represent application of a function a given number of times. In other words:

**0**(f,x) == x

**1**(f,x) == f(x)

**2**(f,x) == f(f(x))

and so on.

The Church numerals figure heavily in combinator theory. The Combinator Engine includes an exposition of them in the **SKI** system.