A piece of salvaged Borg technology with which an older Harry Kim and Chakotay send corrections fifteen years back in time to Seven of Nine, to avert Voyager's destruction.
The temporal transmitter is a Borg device able to send a signal through time to a specific Borg transceiver. It appears in the Voyager episode Timeless.
In an alternate future, Voyager's attempt to fly home with a quantum slipstream drive ended with the ship crashing on an ice planet, killing everyone but Chakotay and Kim, who had flown ahead in the Delta Flyer. Fifteen years later the two men steal the transmitter from Starfleet Intelligence, recover Seven of Nine's remains from the wreck and use her cranial implant to work out the exact moment to target.
With the Doctor's help, and with Captain Geordi La Forge's USS Challenger in pursuit, Kim sends new phase corrections to Seven. The first set fails; a second message shuts the slipstream down safely and erases his own timeline.