Undead is the game’s broad classification for beings that have died yet refuse normal dissolution and still move — the zombie, skeleton, lich, vampire, wight, and wraith all fall under it. Not being alive, they are immune to physiological effects such as poison, sleep, and fatigue, and are bound by the common rule of vulnerability to divine magic (turn undead).
The very definition — "dead yet not ended" — is the widest bridge linking the unsealed dread of cosmic horror to dungeon fantasy.