Source : Free On-Line Dictionary of Computing

extensional equality
     
        (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}.