The term undead refers to either a corpse animated with an unnatural life (hereafter dubbed "corporeal undead"), or the disembodied spirit of a slain person (hereafter dubbed "incorporeal undead"). The term comes from the fact that the undead are neither strictly alive nor dead. Undead may be animated by science, sorcery or even sheer will.
Incorporeal undead, such as ghosts, tend to retain the intellect they had in life, but usually in a fragmentary form.
Some undead gain fantastic powers, while others are powerful enough with their freedom from the limitations of the living, such as pain or the capacity for exhaustion.