A highly combustible gas found in certain nebulae, which Voyager set out to collect as an energy source in Flashback, prompting the buried memory that sent Tuvok back to his days aboard the Excelsior.
Sirillium is an energetic, easily ignited gas that occurs naturally in some nebulae. Starships can gather it for use as a power source, though its volatility calls for care.
It appears in the Voyager episode Flashback in 2373. Voyager finds a nebula rich in sirillium and Janeway decides to harvest some. As the ship approaches, Tuvok is overcome by a vision of a girl falling from a cliff and collapses. The Doctor suspects a repressed memory, and Tuvok mind-melds with Janeway to trace it.
The meld takes them to 2293 and the USS Excelsior under Captain Sulu, where the young Ensign Tuvok served during the events of Star Trek VI: The Undiscovered Country. The Excelsior was then near a similar sirillium-bearing nebula, and the sight of one had stirred the memory, which proves to be a virus disguised as a recollection passing from host to host.