Undead: Technical Def.

The undead hunt the living. What drives a vampire to drink blood, a zombie to eat brains, a wraith to drain life?

They don't have souls, and need the soul energy to sustain their unnatural existence. The only way a 'wild' undead can get it is to kill the living and capture some of the energy as the soul leaves the body: It's kill or die.

This definition also allows for other sorts of undead: One can have an undead truck or ship, as long as they work the same way.

A 'tame' undead is a minion, fed by the minion master's power; see that entry.