Regeneration

Regeneration was the ability to be quickly cured of a wound or to be reconstituted after amputation or destruction of a part of this entity.

Slayer
Slayers could recover from even very severe injuries in remarkably short periods of time. Usually,Buffy was completely healed within 24 hours of being injured, though more serious injuries have taken at least a few days; Buffy suffered from a severe stab wound to the belly that required at least a few days to heal, and, according to Vi, was heavily bruised for several days after her second encounter with the Turok-Han. Also, the Slayers could receive scars; Buffy had a scar left from The Master's bite on her neck, though Angel and Dracula have also bitten her in the exact same spot.

Vampire
Vampires heal rapidly, albeit in a similar way to human bodies (Spike's broken back heals, but after months have passed). Angelus, for example, was beaten several times with a crowbar, recovered in little over a minute and was able to engage in a full-blown sword fight with Buffy without any physical handicap. They can also have unhealable scars under certain conditions. Xin Rong used an enchanted sword to give Spike the scar on his eyebrow. Faith, aided by the spirit of Artemia, gave Kakistos a permanent scar and blinded him in the right eye. Vampires were unable to regenerate lost limbs like Claw though they can be surgically reattached like Spike. The surgical extraction of the heart like James or a powerful object as the Gem of Amara gives them the capacity to regenerate in a few seconds like Harmony Kendall and Marcus.

Demon
If severely injured, Mohras would regenerate and grow both in size and strength. Following the destruction of the Seed of Wonder, and hence the loss of magic, it has been revealed that Mohra blood, instead of simply healing injuries, now triggers perpetual painful regeneration in those who use it, with their bodies resembling massive tumours; Angel and Faith were forced to decapitate a group of people who had tried to use the blood to heal themselves.
 * Even beheaded, the Brotherhood of Seven like Marc and the Deathwok Clan like Lorne have the capacity to resolder their head ; with that, amputated on another being like the Nahdrah Demon ; or to grow a new head like Jenoff.
 * Norman Pfister is, in fact, a sentient hive of maggots, capable of being transformed into a human and giving him the capacity to regenerate at will.
 * If gathered, the body part of The Judge allow him to regenerate.
 * The blood of a [Mohra Demon, known as the "Blood of Eternity", was colored green, glowed and had uncanny regenerative properties, making them virtually immortal. This blood could also heal other beings, and was capable of transforming Angel back into a human being when his blood mixed with the Mohra's.

Witches
Willow has the power to regenerate her skin after she was captured by Gnarl and her face having been deformed by a skinless Warren.

Others being
Ronald Meltzer can control at will his molecular structure, allowing him to remove his hands or eyes of his own body to make them levitate.