The Restoration

The Restoration was the quality which allowed the Eighth Doctor to regenerate in a timeline where he used the Moment to time lock the Last Great Time War and destroy the Time Lords and the Daleks, even though the Moment's nature should have made his existence fixed and prevented him from changing ever again.

On the last day of the Time War, a woman gave it to the Doctor with a kiss shortly before her death. As he regenerated, the Doctor reflected that he might one day pass the gift on to another, (PROSE: Doctor Who and the Time War) and would later save Rose Tyler from the Bad Wolf entity with a similar kiss, that also forced his ninth incarnation to regenerate. (TV: The Parting of the Ways)