A device that lets a small craft open a rift through time, obtained by Admiral Janeway from the Klingon Korath to return to Voyager's past in the finale Endgame.
The chrono deflector is an experimental temporal device developed by the Klingon scientist Korath in the early 25th century of one possible future. Fitted to a shuttle, it generates a temporal rift through which the craft can travel into the past.
It appears in Endgame, the final episode of Voyager. In that future the ship has taken 23 years to get home, at a heavy cost in lives. Admiral Kathryn Janeway bargains with Korath for the device, takes it when he tries to change the terms, and uses it to go back to 2378 and meet her younger self in the Delta Quadrant.
Armed with future technology, the two Janeways guide Voyager home through a Borg transwarp hub, erasing the timeline from which the admiral came.