Immortality is the ability to simply not die by natural means for any number of reasons, ranging from simply having no limit to your lifespan, being able to regenerate from wounds that would normally be lethal, or being protected by a higher being, among other possible reasons. It should be noted that no Immortality is truly perfect, and no matter how much a character's immortality is hyped up within their own setting, it is very, very unlikely that they are truly completely unkillable when matched up against characters from other fictions.
1: Eternal Life: Characters gifted with this type of immortality cannot die from natural causes, such as old age or conventional illness, but can be killed by unnatural causes. To clarify, this type of immortality can include both those who do not age at all, and those who still grow old, but will never die of old age. However, in the case of the latter it should be made clear that this is not just Longevity, as characters with that ability will eventually die of old age, as opposed to Eternal Life, for which dying of old age is not possible.
- Examples: The Valar (Lord of the Rings), Galactus and the Silver Surfer (Marvel Comics), Ajimu Najimi (Medaka Box)
2: Resilient Immortality: Characters with this degree of immortality can survive injuries that would otherwise be lethal to a normal person, without needing to heal from it.
3: Immortality via regeneration: Characters with this type of immortality can simply regenerate from wounds that would normally be lethal, though its effectiveness depends on the degree of the regeneration.
- Examples: Doomsday (DC Comics), Majin Buu (Dragon Ball), Alucard (Hellsing), Hourai Immortals (Touhou Project)
4: Immortality via godhood, or protection from a deity: A character that was either granted immortality by a god, or is immortal because of its hierarchical position due to godhood, so that its divine immortality is less a power, and more treated as a consequence of its state of being as a deity.
- Examples: Future Zamasu (Dragon Ball), Zeref Dragneel (Fairy Tail), Bill Cipher (Gravity Falls), Juggernaut (Marvel Comics), Pontos (Saint Seiya)
5: Deathless Immortality: Characters who exist unbound by conventional life or death, or do not exist at all, and thus cannot be traditionally killed. Typically, abilities such as Existence Erasure are needed to destroy them.
- Examples: All Dark Area Digimon (Digimon), Sun Wukong (Journey to the West), Ultimate Ones (Nasuverse), Hourai Elixir Users (Touhou Project)
6: Parasitic: The character is able to attain a sort of immortality by abandoning bodies whenever necessary to transfer their consciousness to another body, whether they are possessing someone else or switching to a backup body.
- Examples: The Lich (Adventure Time), Teridax (Bionicle), Jason Voorhees (Friday the 13th), Orochimaru (Naruto), Darth Sidious (Star Wars Legends)
7: Undead: Characters who cannot die due to technically being already dead, often overlapping with other forms of immortality.
8: Reliant Immortality: The character cannot die as long as a certain being, object, or even concept exists.
- Examples: Lucemon - reliant on malice in the hearts of men (Digimon), Voldemort - reliant on Horcruxes (Harry Potter), AIM Burst - reliant on the Level Upper Network (To Aru Majutsu No Index), Fairies - reliant on aspects of nature (Touhou Project), Chara - reliant on the desire to kill (Undertale)
9: Transcendental Immortality: Characters whose true selves exist independently from the plane where they can be killed.
10: Meta-Immortality: Entities that are not alive or dead in a conventional sense, standing outside the ordinary laws of reality, temporality, and dimensionality (of any number). If it is possible to destroy such a character, it can only be accomplished by a being of a similar or higher existence.
- Examples: All 1-A characters