Regeneration is accelerated natural healing that happens in battle.
For example, at the start of a new combat round, a character with Regeneration (10%) will regain 10% of their maximum HP (or however much they're missing, whichever's less), rounded up. This even happens while a unit is Diseased.
Regeneration also decreases the chance of getting an Affliction (after the increase from being Cursed but before the decrease from having Affliction Resistance), making it even nicer on thugs:
|Regeneration Percent||Affliction Chance Multiplier|
|5%||1/1 (no change)|
|10%||1/2 (50% less)|
|15%||1/3 (67% less)|
|20%||1/4 (75% less)|
|25%||1/5 (80% less)|
|30%||1/6 (83% less)|
|n%||1/(n ÷ 5)|
A character's starting HP in a fight might not be their actual maximum HP. Regeneration only restores lost HP up to the maximum, and therefore doesn't start healing until after that maximum is plunged beneath.
Regeneration is a Nature buff:
Note that all of these provide Limited Regeneration, which doesn't work on the Inanimate. 10% Unlimited Regeneration is provided by the Reforming Flesh Bless, which requires 6 on your Pretender, but that Bless effect only applies to the Undead.