  We prove some general estimates for exponential sums over subsets of finite
fields which are definable in the language of rings. This generalizes both the
classical exponential sum estimates over varieties over finite fields due to
Weil, Deligne and others, and the result of Chatzidakis, van den Dries and
Macintyre concerning the number of points of those definable sets. As a first
application, there is no formula in the language of rings that defines for
infinitely many primes an ``interval'' in Z/pZ that is neither bounded nor with
bounded complement.
