The Moment

The Moment was a device mentioned by the Partisan that the Doctor planned to use to end the Last Great Time War, destroying Gallifrey, the invading Dalek forces, and the Time Lords, all in a second. (DW: The End of Time)

Presumably, given the device's name, this may also be the device that created the time lock that bound the Last Great Time War from anyone who might be tempted to travel back in time to any point during its duration and tamper with its events.

The Moment might be the modified De-mat Gun that the Doctor created, as the Tenth Doctor stated that, "...once he turned the key in the lock, he doomed them all..." (IDW: The Forgotten)