The Moment

"The Moment" was something with which, or a time at which, the Doctor facilitated the end of the Last Great Time War. In or with "the Moment", the Doctor destroyed Gallifrey, the invading Dalek forces, the Time Lords, and all the horrific entities created during the Time War, as his initial method of foiling Rassilon and his plan to bring about the End of Time. (DW: The End of Time)

The Partisan confirmed to Rassilon and his council that the Doctor was in possession of "the Moment". (DW: The End of Time).

The Doctor would later state that all the combatants of the Time War had burned, "...wiped out in one second...", which suggests the literal suddenness of "the Moment". (DW: Dalek)

Potentially, given the device's name, "the Moment" may have also been the device which created the time lock that sealed the war off from the time vortex. The Moment might have also been the modified De-mat Gun created by the Eighth Doctor, of which the Tenth Doctor said, "...once he turned the key in the lock, he doomed them all..." (IDW: The Forgotten) All this aside, there is nothing to indicate that The Moment was necessarily a piece of technology. Given the nature of the Time War, it could simply have been 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. It is further possible that "the Moment" was a place that could be possessed, a key vantage point which gave him battlefield supremacy.