Regeneration

Regeneration in the Age of Empires series is an ability a unit can have. Units with regeneration ability slowly heal their own HP without the need of a healer, making them valuable in pitched battles and often essential in campaign scenarios where the player cannot build a base and train units, but has to fight their way through with a starting army.

Age of Empires II
In Age of Empires II, there are a few units that can regenerate, most notably heroes. With the release of The Conquerors all heroes gained a regeneration ability, making them very valuable in campaign scenarios. Often, heroes are also must-survive units, and the regeneration ability aids the player greatly since they can actually use the heroes instead of just having to protect them.

Among regular trainable units, regeneration is a fairly exclusive trait. The Berserk is the only unit to have an innate regeneration ability which can be furthered by researching Berserkergang. And Berber Camel troops (Camels and Camel Archers) gain a regeneration ability once Maghrabi Camels is researched.