Undead is a type of classification for monsters or a race for monsters meaning neither dead nor alive, but sort of in-between. Not being dead or alive, and being in between, they have the ability never "die" as they "are already dead", so to speak. They have complete immortality, and some of them are in the line such as vampires, simulacrums like Frankenstein's monsters, like Frankie Stein, mummies, zombies, and many, many more.
They have no real attribute of ability to die, and they can live to be hundreds, or thousands of years—even millions, perhaps. Not having any abilities to die comes with great advantages, meaning they can do anything deadly or risky without dying. Some of them have short limitations, such as zombies, such as only being able to speak in moans and groans, and walking slouched over.