Undead, decaying corpse
Zombies (unofficially classified Type 4) are post-Human supernatural beings which are all but nonexistent and can only be created under certain circumstances. Unlike Ghosts, Zombies still inhabit their bodies, although it decays as time goes on.
When John Mitchell crossed to Purgatory to rescue Annie Sawyer, (Lia (episode)) this caused Sasha and four others who died while Mitchell was on the Other Side to become Zombies instead of cross over. A hospital experimented on the four other Zombies to learn what was keeping them reanimate, while Sasha eventually decayed away after a couple of weeks. (Type 4)
It is known that if and when a being that is not truly dead such as a Vampire crosses to the Other Side, this freezes up Ghosts' Doors until the said non-dead being returns to the physical plane; which in turn means that the spirits of anyone who dies while the non-dead being is on the Other Side, instead of becoming Ghosts and crossing over, will rebound back into their dead bodies and become Zombies.
As Zombies' bodies are simply corpses inhabited by their spirits, they decay over time and any injuries they suffer are permanent, but the Zombies will feel no actual pain. Eventually, after a Zombie's body decays greatly enough, this will apparently negate the force keeping the Zombie reanimated, and the Zombie will die.