From Guild Wars Wiki
- If a spell or skill gets updated to damage or heal for less, cost more to cast, or has a longer recharge, it has been nerfed.
- The change in mob reaction to AoE damage causing them to scatter when in the AoE effect.
- The change in mob mechanics when attacking as a group, especially when a player is solo farming. Some enemies will refrain from fighting to heal then rejoin the fight as other members of their group withdraw.
The opposite of a nerf is a buff.
A nerf is often viewed in a negative light. When changes are made to the game it can make a particular style of play or build that is a part of a farm or a run to become unviable.