The Free On-line Dictionary of Computing (30 December 2018):

Church integer

    A representation of integers as functions invented by
   Alonzo Church, inventor of lambda-calculus.  The integer N
   is represented as a higher-order function which applies a
   given function N times to a given expression.  In the pure
   lambda-calculus there are no constants but numbers can be
   represented by Church integers.

   A Haskell function to return a given Church integer could be
   written:

   	church n = c
   		   where
   		   c f x = if n == 0 then x else c' f (f x)
   			   where
   			   c' = church (n-1)

   A function to turn a Church integer into an ordinary integer:

   	unchurch c = c (+1) 0

   See also von Neumann integer.

   (1994-11-29)