A highly unstable and poisonous waste product of warp engines, valuable only as a weapon, which thieves tried to steal from the Enterprise-D and Sisko used against the Maquis.
Trilithium resin forms in a starship's warp core as an unwanted by-product. It is toxic to humans and violently explosive, so it has no legitimate use, but it would make a terrible weapon in the wrong hands.
In the Next Generation episode Starship Mine, a gang poses as technicians during a baryon sweep of the Enterprise-D in order to drain the resin from the engines and sell it. Picard, alone aboard, fights them deck by deck.
In the Deep Space Nine episode For the Uniform, Sisko answers Michael Eddington's poisoning of Cardassian colonies by firing torpedoes loaded with trilithium resin into the atmosphere of the Maquis world Solosos III, making it uninhabitable for humans for decades and forcing Eddington to surrender.