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, all of the other Time Lords (except for the Master, who had fled to the End of the Universe after the Dalek Emperor took control of the Cruciform), and all the horrific entities created during the Time War, like the Skaro Degradations, the Nightmare Child, the Hordes of Travesty, the Could've Been King and his Army of Meanwhiles and Neverweres.

The Partisan confirmed to Rassilon and his council that the Doctor, "...still possesses the Moment..." (DW: The End of Time), which suggests that the Eighth Doctor had begun his own plan to stop Rassilon and his final solution.

The Doctor would later state that all the combatants of the Time War had been, "...wiped out in one second...", suggesting 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.