What is a Buff in WoW? World of Warcraft Terms

If you have played World of Warcraft for any length of time you have most likely heard party members say things like “buff up” or “give me some buffs”. The word buff in this case simply means any type of friendly spell that lasts for a while. For example, Power Word: Fortitude is a priest buff that increases your stamina, while the paladin spell Holy Light is not a buff because its effects are applied instantly.
What is a buff?