This is the close brotherly relationship between the deceased cured vampire/human doppelgänger, Stefan Salvatore and the vampire, Damon Salvatore. Their relationship over the years has changed from being best friends to enemies because of a vampire named Katherine Pierce. But they start to reconnect their old friendship thanks to Elena, a former vampire who now is a human and Alex Gilbert, a tribrid-doppelgänger.They are Katherine's doppelgängers.
Their relationship has improved throughout the series, as they have always been willing to do anything for each other. In Season 5, they appear to be getting along better as brothers, as they appear to have much more intimate moments than they used to before. In the Season 5 finale, Damon gave up his life to save Stefan, which devastates him.
In The World Has Turned and Left Me Here, Damon returned and met Stefan in their family's crypt, where the brothers embraced for the first time since 1864.
Their bromance is known as "Defan".