Regeneration (or regen) in Diablo III generally refers to the ability (whether native or buffed by item modifiers) to regenerate a resource, whether it be health or the mana-equivalent resource of any of the five classes. It can also refer to a monster's ability to regenerate its own health over time, as was frequently used in Diablo II.
Willpower is a character attribute that may end up boosting the regeneration rate of each character's respective resource. More will be known about regeneration as the beta gets underway. In addition, some spells or skills may aid in regeneration, such as the Monk's Holy Aura.
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