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

constant applicative form
CAF

    (CAF) A supercombinator which is
   not a lambda abstraction.  This includes truly constant
   expressions such as 12, (+ 1 2), [1, 2, 3] as well as partially
   applied functions such as (+ 4).  Note that this last example
   is equivalent under eta abstraction to \ x . + 4 x which is
   not a CAF.

   Since a CAF is a supercombinator, it contains no free
   variables.  Moreover, since it is not a lambda abstraction it
   contains no variables at all.  It may however contain
   identifiers which refer to other CAFs, e.g.

   	c 3 where c = (* 2).

   A CAF can always be lifted to the top level of the program.
   It can either be compiled to a piece of graph which will be
   shared by all uses or to some shared code which will overwrite
   itself with some graph the first time it is evaluated.  A CAF
   such as

   	ints = from 1  where  from n = n : from (n+1)

   can grow without bound but may only be accessible from within
   the code of one or more functions.  In order for the garbage
   collector to be able to reclaim such structures, we associate
   with each function a list of the CAFs to which it refers.
   When garbage collecting a reference to the function we collect
   the CAFs on its list.

   [The Implementation of Functional Programming Languages, Simon
   Peyton Jones
(http://research.microsoft.com/%7Esimonpj/papers/slpj-book-1987/PAGES/224.HTM)].

   (2006-10-12)