Decapitation is the act of removing and/or cutting a person head from their body. This is fatal weakness to Vampires.
Like in the TV Show Supernatural, Vampires can be killed by cutting their head off. However unlike the vampires in Supernatural, Vampires can be revived if there head is placed back on their body, forcing their healing factor to reattach the head back on the body.
Theo Quinn: Theo Quinn's healing process has an amazing regenerative abilities, enabling decapitation to not kill him, as his head reattaches itself within seconds.