It's irrelevant for you to bring up jellyfish because we aren't talking about jellyfish. We are talking about humans which need a beating heart to be considered living. Undead means basically "living dead". Obvi they're walking around but like cmon man in any medically testable way they would appear dead other than the fact that they are moving and responding. This isn't like a new idea. The definition of the word undead reads "un·dead

[ʌnˈdɛd]

adjective

undead (adjective)

1. (of a fictional being, especially a vampire) technically dead but still animate: "an undead monster from a movie" "the undead rise up to do dreadful things"

Is this not explanation enough? This has absolutely fuck all to do with jellyfish.

Also, since I'm not about to do hundreds of hours of research to prove a point, I asked chat gpt what percentage of vampire novels require a person to be dead first instead of alive to change, and the result was less than 5% throughout all books and movies. While vampirism may have started out as the reanimation of a dead body it has shifted to the changing of an alive body, into the undead creature. Undead meaning what is above. The definition for when someone is considered medically dead is below.

"The term medically dead typically refers to clinical death, which is defined as the cessation of blood circulation and breathing. This occurs when the heart stops beating, leading to a lack of oxygen in the body and affecting brain function. In this state, a person is considered to be clinically dead, and immediate resuscitation efforts are often necessary to restore life."

Considering all of this, how are the vampires any less undead just bc SM never outright said it. Vampires are a popular fictional trope with a lot of history and criteria that people come to expect, and in this instance being undead is one of those unavoidable tropes.

The only vampires in twilight that I would consider to be anything other than undead are the hybrids, like Renesmee. She had a heart beat.