Empower is a class of damage buffs you will have to look up which sources give them.

[quote]What does that mean?[/quote] Empowered Weapons: witherhoard, and osteo striga are empowered weapons. They do continuous damage after you stop firing the weapon. Some say Thorn, Necrochasm, and Touch of Malice are empowered too, since they’re Weapons of Sorrow. Supposedly, any of the “Weapons of Sorrow” are considered empowered weapons. But Witherhoard and Osteo work for sure.

Things that empower the weapon like a warlock "empowering" rift.