A dead witch is a type of witch which lives on after death. The soul of a witch is "bone bound," and the witch lives on as a slowly-decaying corpse. A witch may be truly killed and sent to the dark in two ways: burning her body or eating her heart. Otherwise, the witch will generally reanimate at the first full moon after she is buried.
John Gregory and some other spooks bury a dead witch upside down, to prevent the witch from digging her way to the surface; instead, the witch is tricked into digging a deeper grave. Bill Arkwright instead feeds the heart of a dead witch to his dogs to prevent her from ever reanimating.
It is implied that all witches are bone bound and can reanimate, but only human witches from the County are shown doing so.
The power of a dead witch depends on her power when she was alive. The witch assassin Needle was able to run and fight years after her death, while the healer Agnes could barely crawl. Dead witches do not have access to magic, but have a great thirst for blood. In time their mind changes: they will forget their memories of life, whom their loved ones are and who their enemies are. Some dead witches are slow and immobile, but all are very strong. Dead witches slowly rot and decay until they are sent to the dark.