Regeneration

From Wikihack

Regeneration is a property that speeds the recovery of HP drastically; your character gains 1 HP on every turn (s)he would not normally do so.

It is acquired as an extrinsic thorugh putting on a ring of regeneration or wielding the Healer's quest artifact, the Staff of Aesculapius. It can only be acquired as an intrinsic by eating a ring of regeneration if the ring is of suitable material and you are polymorphed in to a monster that can eat it.

The downside to having faster regeneration however, is if the property is from a ring, either eaten or worn, you lose an additional point of nutrition every other turn.