Regeneration (also known as healing factor) is a term given when a particular character can heal from wounds at an accelerated rate, even regenerating from wounds that would be lethal to normal humans. Regeneration has different levels.
Low levels of regeneration are essentially just faster natural healing, so a wound that could take weeks to heal would take days, hours, or even seconds. More advanced forms of regeneration can regrow lost limbs, organs, going all the way up to regenerating from decapitation, being blown to pieces or vaporized, and even complete physical destruction.
Low: Faster healing ability for normal wounds. In machines and vehicles, this would be regenerating basic exterior damage.
Mid-Low: Able to heal wounds that would normally leave large scars. In machines and vehicles, this would be regenerating damage that would normally leave large dents and openings.
High-Low: Able to regenerate some severed body parts like fingers, arms, legs. In machines and vehicles, this would be regenerating some interior damage, and some minor critically damaged or destroyed parts.
Low-Mid: Able to regenerate limbs/fatal wounds. Still die from decapitation. In machines and vehicles, this would be regenerating minor parts and more extensive internal damage.
- Examples: Shirou Emiya through Avalon (Fate/Stay Night), Jason Voorhees (Friday the 13th), Wolverine (Marvel Comics)
Mid: Able to regenerate from decapitation. In machines and vehicles, this would be regenerating from the destruction of critical parts such as the engine.
High-Mid: Able to regenerate from being blown into pieces.
Low-High: Able to regenerate from a puddle/drop of blood, or a small piece of your body.
- Examples: Cell (Dragon Ball), Homunculi (Fullmetal Alchemist), Alucard (Hellsing), Ban (Nanatsu no Taizai)
Mid-High: Able to regenerate from being vaporized/dust.
High: Able to regenerate from a single molecule/atom/particle.
- Examples: Anthrax (Bastard!!), Doctor Manhattan (DC Comics), The Sentry (Marvel Comics), Kakine Teitoku (To Aru Majutsu No Index)
Low-Godly: Able to regenerate as long as your disembodied consciousness exists, such as in the form of your soul or mind, or from other realms.
Mid-Godly: Able to regenerate even after every aspect of the mind, body, soul, and consciousness, are erased from existence.
High-Godly: Able to regenerate even if your existence is erased and the reality you are within is destroyed.
True Godly: Able to recover after being erased from both existence and nonexistence. Only characters that can regenerate from a beyond-dimensional (Tier 1-A) substance can qualify for this level.
To qualify for True Godly regeneration, a character must be able to regenerate from being erased from the duality of both existence and nonexistence, whereas it is only necessary to be able to recover from the former in order to warrant a Mid-Godly or High-Godly rating.
1-A entities (beyond the concepts of dimensions of space and time) always have High-Godly regeneration (given that they are above existence per definition), but not automatically the highest rating. In order to qualify, such beings must also have displayed the capacity to recover from complete outerversal destruction, or be logically capable of doing so.