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

extensional equality
extensionality

   (Or extensionality).  Functions, f and g are extensionally
   equal if and only if

   	f x = g x  for all x.

   where "=" means both expressions fail to terminate (under some
   given reduction strategy) or they both terminate with the
   same basic value.

   Two functions may be extensionally equal but not
   inter-convertible (neither is reducible to the other).  E.g.
   \ x . x+x and \ x . 2*x.  See also observational
   equivalence, referential transparency.