A Borg propulsion component that lets a vessel open and travel through transwarp conduits, one of which the crew of Voyager stole from a damaged sphere in 2375 to shorten their journey home.
A transwarp coil is the heart of Borg faster-than-warp propulsion. Every Borg vessel carries at least one, and it allows the ship to enter transwarp conduits and cross thousands of light-years in a short time.
In the Voyager two-parter Dark Frontier, Captain Janeway plans a heist to take a coil from a sphere crippled by an ion storm. The raid succeeds, but Seven of Nine stays behind and is taken to the Borg Queen. A coil fitted to the Delta Flyer lets Janeway follow and bring her back.
Once installed aboard Voyager, the stolen coil carries the ship about 20,000 light-years closer to the Alpha Quadrant before burning out, cutting roughly fifteen years from the voyage.