The Prolog Language
Prolog (for PROgramming in LOGic) is a logic programming language loosely based on first-order logic. It was invented by Alain Colmerauer and Phillipe Roussel at the University of Aix-Marseille in 1971. Since then a large number of good implementations of the language have become available.
See http://vl.fmnet.info/logic-prog/ for more details.