In regards to online games, specifically those of the massively multiplayer "persistent world" type (Everquest, Asheron's Call, Ultima Online, and so on), 'to nerf' refers to the act of making a game element weaker than it was previously.

Originally, 'to nerf' was used exclusively to the weakening of player characters and was derogatory - the metaphor being that a player's "real" swords, armor, et cetra had been replaced with foam Nerf items, and the implication that it was not needed. Now, the term is more general and refers to any change, including those considered desirable for game balance reasons. For example, "In Everquest the spell resistance rates of the high-level monsters was just nerfed."

The reverse of 'nerfing' is to buff.