The Moment

The Moment was a piece of technology that the Doctor used as a weapon of catastrophic proportions to end the Last Great Time War (DW: The End of Time). The Moment destroyed Gallifrey, the invading Dalek forces, the Time Lords, and all the horrific entities created during the Time War. The Doctor would later relate that they had burned, all in a second. (DW: Dalek). The Time Lords were aware that the Doctor had the Moment; the Partisan reported as much to Rassilon and the inner council of the Time Lords (DW: The End of Time).

Potentially, 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 the documented events of the war.

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)

However, there is nothing to indicate that The Moment was a piece of technology. Given the nature of the Time War, it could simply be a literal moment; a period of time, with a brief but indefinite, length within which the Doctor had the opportunity to do whatever he did to end the Last Great Time War.