(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}.