Trilithium is a element acting as inhibitor for nuclear reactions like they occur inside stars in form of nuclear fusion.
It was used by Dr. Tolian Soran to stop the nuclear reactions that took place at the heart of the star Amargosa in order to change the course of the Nexus. Dr. Soran procured his Trilithium from the Duras sisters, who in turn stole the Trilithium from the Romulans. (Star Trek: Generations)