An Aura is an AoE skill that can provide a constant buff to your party members or debuff to your enemies.
Diablo III[]
Auras will be usable by the Monk class in Diablo III. However, unlike the Paladin's auras in Diablo II, they will have a short duration and must be reactivated to provide their benefits.[1] In some cases they may also have instant effects.[2]
Diablo II[]
Auras in Diablo II are primarily used by the Paladin class, but can also be granted by special equipment and hirelings. If the aura is granted from a skill, the Paladin can only have one such aura active at a time. Multiple auras are stackable if they originate from different sources such as other members in the party, their mercenaries, or their equipment.