A nerf is a term used to describe the weakening of a character or an aspect of a character, either through updating a game or by releasing a sequel. Nerfs are often done as an attempt to balance the roster, by weakening characters that are too strong; however, nerfs can also be inadvertently added if a character is indirectly weakened by an external gameplay change. Nerfs contrast with "buffs", which involve the strengthening of a character or aspects of a character between sequel games and game updates.
The term "nerf" is derived from the brand of toy known for its soft foam-like substance. This substance allows for safer play, as little chance of injury comes from impact with such items. In effect, a projectile is less damaging because it was "nerfed".
Notable examples of nerfs in the Super Smash Bros. series
Smash 64 to Melee
Melee to Brawl
Brawl to Smash 4
Smash 4 game updates
Smash 4 to Ultimate
Ultimate game updates