A nerf is a term used to describe a decrease or weakening over a character's aspects and attributes, which may come, either, by game updates or by releasing a new game. It is often done for balancing purposes, characters that may prove to be highly effective, to the point of been considered broken or unfair against other characters receive nerfs to compensate these problems. Its opposite term is buff, which means strengthening a character between games and updates.
While the transition of characters between Super Smash Flash and Super Smash Flash 2 saw a huge buff, this did not exonerate some attacks to be nerfed, the most notable being the projectiles, who were considered overwhelming in SSF for all characters, whereas in SSF2 projectiles are coded to work according to the character, changing its effectiveness. With new demo releases for SSF2 also come new nerfs for certain characters, handled by the balancing team.