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

fixed point
fixpoint

    The fixed point of a function, f is any value, x
   for which f x = x.  A function may have any number of fixed
   points from none (e.g. f x = x+1) to infinitely many (e.g. f x
   = x).  The fixed point combinator, written as either "fix"
   or "Y" will return the fixed point of a function.

   See also least fixed point.

   (1995-04-13)