The thing that flows in a vampire's veins, sometimes mistaken for blood.
Sisko talked a lot about it: how it is not made of cells, or atoms, or any other particles; how it vanishes when it burns, loosing way more mass than the energy it gives of in the process; and how it is the many hopscotching meta-vectors of a virus, infecting reality with seemingly less advanced versions of themselves.
Strong Vitae can recover a vampire from Frenzy or Torpor. Kindred can sustain themselves on Vitae, but it's really strong stuff, and it creates a blood-bond, enslaving the drinker to the drunk.