An isolitic converter was an engineering tool which could be used to unfuse engine controls.
On the Silver Blood USS Voyager in 2375, before going on her honeymoon, B'Elanna Torres warned Seven of Nine that the isolitic converter was "temperamental". Later when the vessel was falling apart, Harry Kim ordered Seven to unfuse the vessel's engine controls, but she stated that she could not comply without an isolitic converter. (VOY: "Course: Oblivion")