The name for types of supernatural beings that are deceased yet behave as if alive. Undead can refer to the spiritual, as in ghosts or the corporeal as in animated corpses. Bram Stoker was the first to use the term "The Un-Dead" as the original title for his novel Dracula.
Dean: You're joking. Then how do they stay alive? Or undead, or whatever the hell they are.
Dean: Um, okay, besides silver, we have ... nailing the undead back into their gravebeds. It's mentioned a few times. It's probably where the whole vampire staking lore came from.
Dean: "And"? And you're welcome – for catching the undead killer zombie.
Dean: So you just want to guide me out of Purgatory out of the goodness of your undead heart?
Dean: Listen to me, you undead blood junkie, I'm the one with the mojo. I'm the one with the plan. Cas... we're gonna shove your ass back through the eye of that needle if it kills all three of us.
Sam: So, what about you? I mean, you're not... undead. So what did you do? You cast a "forever 21" spell on yourself, like your little friend at the library?
Dean: Well, now we know – paper beats Golem, fire beats undead Nazi zombie freaks.
Bobby: Undead creature that feeds on souls. Hence the name. They exist in a place between our world and… another. Soul Eater moves into a house, and once it does, it makes what the lore refers to as a nest.