Brief Summary
Immortality is a term for characters that have the inability to die through conventional means alone.
Types
1: Biological. Characters with this immortality are incapable of dying through aging and senescence. In some cases they're also uncapable of dying through disease, though not always, unless otherwise stated—though the statement isn't necessarily required, it makes determining it far easier. Not to be confused with Longevity.
2: Resilience. Characters with this immortality are highly resilient towards physical attacks, but can't regenerate from it. For example, the character will be still alive if he's cleaved in half or split into pieces, but he can't paste himself back together. Severed limbs won't grow back, nor will destroyed organs.
3: Regeneration. Characters with this can simply regenerate from their wounds. The usefulness of this is all dependent on the user's degree of Regeneration.
4: Godhood or God Protection. Characters with this have been granted immortality through godhood, or through divine intervention. This is generally more so treated as a state of being rather than an actual immortality.
5: Absolute. Characters with this have literal incapability of dying whatsoever. It's generally reserved for Omnipotent beings, rather than those much lower; although some exceptions DO exist, albeit very, very, very rare.
6: Parasitic. Characters with this immortality are capable of bodyhopping or utilizing Possession to stay alive, moving their spirit, soul, essence, concept, mind, consciousness or embodiment to another body to take control of. More rarely, they can even take control of objects.
7: Undead. For the most part, this immortality is self-explanatory, and characters with it have a habit of doubling up with other types of immortality. A common one would be Type 1, and for ghosts, they normally end up having Type 3 as well.
8: Reliant. Characters with this immortality are incapable of being killed so long as a certain object, artifact, phenomenon, concept or something else exists or is intact, they cannot die during a certain situation or circumstance or for as long as a certain emotion exists towards them or they exist for only one purpose (anymore). If the thing is broken, destroyed, killed or deactivated, the situation ends or the purpose has been fullfilled, the character will either die or become mortal.
9: Transcendental. Characters with this immortality are actually separate from the realm where they can be killed. For example, a conceptual being doesn't die even if its body, soul, etc. are erased from existence. Usually, these are higher-dimensional forces or characters with a lower-dimensional avatar/manifestation.
10: Meta. Characters with this immortality are not dead, nor are they alive. They are completely metaphysical, and thus are outside of reality, temporality and dimensionality of any measurement. If possible to kill such a character, you would need to be on a similar plane of power, or higher.
11: Force. Characters with this immortality are a force with a mortal manifestation or become a force upon death that isn't truly sentient and cannot perceive, feel, think, see, hear, smell or taste, but can continue to perform tasks after their passing. A good example of this is Dimentio from Super Paper Mario, who left a shadow of his power to resume destroying worlds even after his opponents had technically already won.
12: Retroactive. Characters with this immortality can be killed--it's just that, one way or another, they'll come back. Normally associated with Resurrection, though users may or can also reincarnate in another body, wether it be the same species or not.
13: Mechanic. Characters with this immortality will always be robots or cyborgs, and they can be killed, but will always be capable of being rebuilt and/or repaired.
14: Selective. Characters with this immortality can only be truly killed by one person, weapon or attack. There are two types; the first, weaker type is the one where these characters can still be defeated, it's just FAR more difficult to KILL them without their specific weakness. The second and strongest type is that they can literally only be killed with/by that one thing and NOTHING else.
15: Physical. Characters with this immortality are completely incapable of dying through physical means alone. They can never be killed through means that would leave a corpse, such as suffocation, injury, impalement and contaminants. Only by complete and utter destruction can these characters be defeated. A good way to kill them would be through Reality Warping or attacks from a higher dimensional being. Characters with this type of immortality usually have regeneration but not always.
16: Multiple Lives. The same as Type 12, but the characters in question can only come back for a limited number of times, having multiple lives. Wether the user can regain lives, get extra lives, how many they have and wether they return in their actual body or in another all depends on the character.
17: Duplicative. A specialized type of immortality. Once the user dies, they'll create a new body or avatar for them to manifest themselves in. This can be a new body, a clone of an existing body, a bionic body, a cyborg body or other "bodies" such as shapes, energies, liquids, essences, forces, antropomorphic objects, etc...
18: Transformation Resurrective. The final variant of Type 12. The character in question will return in another form or one of their transformations or other forms. It is not uncommon for character to be unable to revert or transform into forms that are already "dead", i.e. forms/transformations that they have already been defeated in - then again, this does not always apply.
19: Mecha-Parasitic. Characters with this immortality are generally machines and/or computer virus characters, capable of uploading their own data to other software or networks to avoid termination.
20: Other Reason. Characters with this type of immortality have a type of immortality not yet described above. The reason(s) why they can't die or description must be stated on the pages of characters with this immortality.