A nerf, also known as nerfing, is a term used to describe the weakening of a character (or aspect of a character), either through updating a game or by releasing a sequel. It is often done as an attempt to balance the roster. The opposite term is to "buff" a character.
The best example of this is Kirby's transition from the original Super Smash Bros. to Melee: Kirby went from being one of the best (tier-wise) characters to being one of the worst. Another example is Peach's transition from Melee to Brawl: Her Forward Throw and Down Smash have been weakened, and she cannot double-jump cancel anymore.
"Nerf" as a verb has been in common use when talking about character or attack changes in all kinds of games, from RPGs to FPSs.
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 has been "nerfed."
Characters who had attributes nerfed from Super Smash Bros. to Super Smash Bros. Melee
There have been four general nerfs for all characters. The first is that not all aerials have almost instantaneous start-up, and the second is that grabs and throws have been significantly weakened, most no longer being KO tools. The third nerf is that smashes have to be charged to have decent KO power, and some smashes have no KO power whatsoever, and the fourth nerf is that not all tilts have instant start up.
His up air has less range and the latter half of the hitbox is much weaker than hitting with the initial part of the attack, which makes semi-spiking much less effective. His new Up Smash is slower and has drastically reduced utility. The hitbox of his D-air is much shorter in duration, has shorter horizontal reach in his N-air (previously his F-air) and Forward Smash, F-tilt much less useful, U-tilt now hits only once, sourspot added to his back air. Falcon Dive is no longer a reliable KO move, and finally, Falcon Punch is slower. Note: Captain Falcon`s most major buff was his new F-Air, The Knee, which makes up for every one of these nerfs.
His Giant Punch has less reach and less knockback, and puts him in a helpless state when done in midair. Although he was arguably the strongest character in the the original, he now has above-average attacks overall (Despite the fact that he looks bigger and stronger).
His Down Smash doesn't semi-spike as well, his U-air is more difficult to sweetspot, and his blaster no longer produces hitstun. He is also significantly lighter, so it's very easy to KO him under 100% damage, even 65% or rarely 60%, depending on the character's strength. His air speed was drastically reduced (went from having among the fastest air speed in SSB, to being tied with Falco for the slowest in Melee).
Arguably the most drastic nerf in a Smash Bros. game. He has much less power, speed, and reach in his attacks; his horizontal air speed is drastically slower; and he is easier to combo; thus making a character with terrible stats in Melee. The only way to match up to his power in Smash 64 is to fully charge his smashes.
Link's Up Smash has a smaller hitbox. His attacks are also slightly weaker overall.
His air speed is slower; the Super Jump Punch lost its horizontal distance, and the height gained from Luigi Cyclone has been lowered (this results in it being more difficult to incorporate these abilities into a horizontal recovery). Green Fireball size has also been decreased.
Super Jump Punch's jumping distance has been reduced. His Up Smash is slightly weaker. Fireball size has also been decreased.
He has less speed and reach in his attacks. His Up Smash and Down Smash are much weaker. His Forward Smash is also much slower.
It has less reach in its attacks, it is now vulnerable to chain throws and Thunder is no longer infinite.
Her dashing speed is slower; her Screw Attack no longer traps enemies, and her charge shot no longer has infinite range.
His specials are generally less effective; his combo ability against floaty characters has been hindered.
Many attacks, such as his Forward Smash and U-air, are slightly weaker in terms of knockback.
Many of his attacks underwent several nerfs in speed, power, shieldstun and priority: the most dramatic examples being D-air, F-air, and Forward Smash. D-air has a much smaller hitbox, no longer being disjointed, can no longer nipple spike, does not Meteor Smash in the upper hitbox, and is weaker in knockback. F-air is much harder to sweetspot (due to a 1 frame sweetspot duration and only a portion of the hitbox being the sweetspot), hitlag was increased to nearly a full second (which makes it easier to DI), and is slightly weaker in terms of knockback. Forward Smash (along with Up Smash to a lesser degree) was weakened severely (from being the 3rd most powerful overall to the 19th most powerful) in addition to being slower. His traction is lower. Dash speed was reduced slightly (to increase the contrast with Sonic). Falcon Kick no longer renews his double jump after it has already been used. Falcon Punch has less knockback. He is overall a slower and much weaker character. He is considered the most dramatically nerfed character in Brawl along with Ganondorf for both the Brawl physics (no L-canceling, no Moonwalking, less hitstun, and less effective dash dance result in much worse combos, though perhaps the largest factor is the all around buffed shield and dodge mechanics) and the major nerfs he received.
Nerfed in many of his KO moves. Forward Smash is slower and needs to sweetspot to be a good KO move. N-air is no longer a KO move. D-Tilt does not KO until extremely high damages. Falco Phantasm's Meteor Smash is weaker. D-air only spikes at the beginning of the attack. Fire Bird travels a significantly shorter distance, is now a multi-hit attack, and is much weaker, being now incapable of KOing, his Reflector can no longer be held, thus, it reflects only one side, and he has to time his reflector right, and thus his shine combos no longer exist. His survivability was also nerfed in Brawl due to the addition of fallweight, which prevents him from taking vertical hits as well as he did in Melee (he will not last anywhere close to how long Bowser can last in Brawl vertically, like he could in Melee).
Shine combos no longer exist. Up Smash is slightly weaker (it is however, often considered the best u-smash in Brawl); U-air is much weaker, up to no longer KOing under 150% reliably, B-air is much slower. Fire Fox is weaker and can no longer KO; and his Blaster travels roughly half the distance it did in Melee. Due to the addition of fallweight, he can no longer survive vertical hits to high percentages and he dies even sooner horizontally, therefore nerfing his survivability in Brawl. Reflector comes out slightly slower.
Slower attacks (most notably jab) and dash speed, leading to very poor approach options, shorter grab (now the shortest standing grab in the game, which combined with all his ground moves being slow makes his Out of Shield options terrible), lower jumping height, poorer mobility, Wizard's Foot no longer enables a midair jump if one was used already, which significantly nerfs his recovery (went from a long recovery to one of the shortest horizontal recoveries in the game). A large amount of his killers were weakened significantly in Brawl, such as his Down Smash, B-air, N-air, Dark Dive, and the grounded Wizard's Foot. His U-tilt was also weakened significantly (but it is still the strongest tilt and it is much easier to land), and the Up Smash was weakened as well (it is still very powerful). Overall, he is a weaker and significantly slower character, especially due to the loss of L-Canceling, and he has a harder time landing KOs due to having less rounded power (more attacks were weakened than strengthen, often on a higher scale) and harder to land attacks.
Power has been reduced significantly. Now are easier to separate. Freeze Glitch has been removed.
Reduced combo ability. Rest is a lot weaker in terms of knockback. Three aerial attacks weakened, most notably its B-air. Forward Smash is weaker, smashes are slower and have longer ending lag. Now ranks the slowest dash speed in the game, and has been seconded by Yoshi for highest air speed. D-air no longer can Meteor Smash, and less hitstun has made WoP harder to pull off.
F-throw and B-Throw no longer Kirbycide. U-Throw can only Kirbycide on certain stages. Swallowcide now KOs Kirby and his victim at the same time.
Much slower air speed, shorter grappling device, those makes his edgeguarding ability much worse, infinite sword stab in neutral combo removed, Spin Attack no longer semi-spikes, and is worse for recovery. His Boomerang, replaced with Gale Boomerang, has lost range and speed, and doesn't deal damage when returning, instead pulling foes with it, and it's worse at comboing. He is overall slower and is much worse in recovering (could be the only character along Ganondorf who has a harder time recovering in Brawl).
F-air and D-air are much weaker. Super Jump Punch has more ending lag. Fireball's range is shorter. Loss of wavedashing has made approach much harder. Misfire travels a shorter distance, making it less useful as a recovery move.
The power and range of many of his moves have been slightly decreased. His F-Air now needs to sweetspot to Meteor Smash and is weaker (But it is still average). Fireball's range reduced. Replaced Mario Tornado with F.L.U.D.D., reducing his overall recovery. Because of Brawl's new autosnap feature, Cape is less useful. His F-Smash's sourspot is much larger (Although it is much stronger). His Neutral Air is weaker and has slightly less range due to his character model being smaller (which is another flaw).
Shorter grab range and shorter sword, therefore shorter range overall. Tipping his attacks is more difficult, such as D-air. Neutral attack combo is slower. Dash speed slightly reduced. Shield Breaker is no longer as useful for edgeguarding and it has lost overhead range and power as it is now a stab instead of an overhead swing. Spike has to be sweetspotted, and lack of wavedashing has resulted in a worse approach.
D-Tilt is weaker. Neutral attack is now a trapping move instead of a KO move. Oil Panic maximum damage reduced from 200% to 60%. Since the wind effect of U-air is added, connecting it with a 2nd attack is harder.
F-Smash is slower and has shorter reach, making it less useful in Brawl. PK Flash is harder to control. The start of his Up and Down smashes are much weaker.
Shorter midair jump (now the shortest in the game), decreased power in f-throw, Down Smash completely nerfed in every aspect, especially in KO power and damage output (can no longer KO and it went from being the best Down Smash to arguably the worst, could be considered the single most nerfed attack in the series). Substantially reduced air speed. Can no longer Double Jump Cancel.
Projectile game and finishers drastically weakened. Missiles are slower and drastically weaker (could KO under 150% in Melee but can't even KO at 300% in Brawl). Charge Shot is significantly weaker (could KO under 80% in Melee but can't KO under 125% reliably in Brawl). N-air was significantly weakened (went from being the second strongest sex kick to one of the weakest). Her bombs are less effective for recovery, and now no longer explode when in contact with opponents, only after their internal timer is finished.
D-Throw is now completely incapable of chain throwing. All of her aerials were weakened, F-air's semi-spike was weakened significantly; Smash Attacks (except the second hit of her Forward Smash) are much weaker and Up Smash's sourspot can no longer KO reliably. The addition of fallweight has also hurt her survivability, she only slightly outlasts Zelda vertically and she is outlasted by Zelda when knockbacked horizontally.
Egg Roll can no longer can be used for recovery. Smash Attacks are weaker (particularly Down Smash). F-air now needs to sweetspot to meteor smash and is much weaker (is now only an average Meteor Smash while it was the second strongest meteor in Melee). Yoshi Bomb and Egg Throw have lower knockback. He can no longer Double Jump Cancel. Egg Lay's KO potential is lower because of the slow falling speed of the egg and higher starting height. Due to Brawl's Air Dodge being unable to stop in the air like in Melee, his recovery is slightly worse.