A nuclear inhibitor was a substance that inhibited nuclear reactions.
Trilithium was a nuclear inhibitor capable of stopping all fusion within a star. In 2371, Tolian Soran, an El-Aurian scientist, utilized trilithium to destroy the Amargosa star as part of his plot to alter the course of the Nexus energy ribbon. The trilithium caused a quantum implosion within the star, resulting in a level-12 shock wave that destroyed everything in the system. Soran attempted to perform the same action on the Veridian star, but was stopped by Jean-Luc Picard and James T. Kirk. (Star Trek Generations)