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

fusion
vertical loop combination

    A program transformation where a composition
   of two functions is replaced by in-lining them and combining
   their bodies.  E.g.

   	f x = g (h x)	==>	f x = g (2 * x)
   	g x = x + 1		f x = 2 * x + 1
   	h x = 2 * x

   This has the beneficial effect of reducing the number of
   function calls.  It can be especially useful where the
   intermediate result is a large data structure which can be
   eliminated.

   See also vertical loop combination.

   (1994-12-05)