Regeneration
The creature regains Hit Points at the start of its turn, as with fast healing, but it can’t die as long as its regeneration is still functioning (although creatures with regeneration still fall unconscious when their Hit Points reach 0). Certain attacks, typically those that deal acid or fire damage, cause a creature’s regeneration to stop functioning for 1 round. During this round, the creature doesn’t regain Hit Points and can die normally. The creature’s stat block describes the types of damage that suppress the regeneration.
Regeneration doesn’t restore Hit Points lost from starvation, thirst, or suffocation. Creatures with regeneration can regrow lost portions of their bodies and can reattach severed body parts if they’re recovered within 1 hour of severing. Severed parts that aren’t reattached wither and decompose normally.
A creature usually must have a Constitution score or modifier to have this ability.
Format: Defensive Abilities regeneration 5 (acid).
Source
Alien Archive pg. 156