Undeath, or unlife as it is sometimes referred to, is a word that is used nearly exclusively in regards to vampires and vampiric lore, as it describes a state of being or the existence of an individual that is deceased but still acting in a lifelike manner.

To be undead, a person first has to die. The way in which a person dies is oftentimes a key component in achieving undeath. For instance, numerous cultures believe that if people commit suicide or die while giving birth, they are primed to return to the world of the living as vampiric REVENANTS, beings who exists in a state of undeath. These individuals are clearly deceased, but nevertheless are still active, mobile, and living out an agenda. Depending on the culture, the conditions that can cause a corpse to rise up in a state of undeath varies. The condition of the body and the mindset of the being that has returned varies as well. Most undead beings and creatures are revenants — animated corpses who seek out blood or vital human essences in order to maintain their very existence. Beings who are undead will not die of natural causes and will “live on” until they are destroyed in the very exacting method that their culture or mythology has predetermined.

Source: Adams, Slayer Slang, 225; Auerbach, Our Vampires, Ourselves, 3-7; Russo, Vampire Nation, 14-16