A spike is a type of attack in the Super Smash Bros. Melee that sends the opponent downward until the hitstun wears off. A spike is not to be confused with a meteor smash, which also sends the opponent downward, but the knockback of which can be cancelled before the hitstun wears off by meteor cancelling.
When referring to Super Smash Bros. and Brawl, "spike" and "meteor smash" are often used interchangeably, as all downwards-hitting attacks in both games are meteor smashes.
In Melee, hitboxes that send opponents at an angle between 260 and 280 degrees are recognised by the game as meteor smashes. However, there exist some hitboxes that send opponents downward, but at a diagonal trajectory outside this angle window (for example, 290 degrees). As a result, the game does not recognise them as meteor smashes, and are thus unable to be meteor cancelled. In Brawl, the angle window for what the game recognises as meteor smashes was increased to being between 230 and 310 degrees, enough so to cover the downward diagonal trajectory the spikes in Melee had. As a result, the spiking hitboxes in Melee that retained their trajectories are recognised as meteor smashes in Brawl, and are capable of being meteor cancelled.
Spikes have the benefits of meteor smashes: knocking the opponent downward with gravity assisting the knockback. But they do so without the drawback of the opponent having the ability to completely negate the knockback with a jump or recovery move. As a result, spikes are extremely effective KO moves in edge guarding situations - recovering from even moderately powerful spikes is usually impossible above extremely low percents. Because of this, many smashers who main characters with practical spikes try to integrate spikes into play as often as possible.
List of spikes in Super Smash Bros. Melee
Characters with useable spikes
Spikes in Super Smash Bros. Brawl
It is technically possible for spikes to exist in unmodified Brawl; the increased meteor smash angle window does not include all downward angles. Additionally, if the angle of a hitbox in Brawl is hacked to outside the standard 0-360 range of angles (for example, 630, which would equal a meteor smash angle of 270), the game will not recognize it as a meteor smash, and it will therefore act as a spike.
There are a few boss hitboxes with angles in the -70 to -90 range, which while unconfirmed is very likely not to be detected as a meteor smash. This includes Petey's jumps and the start of Rayquaza's digging and explosion attacks. Aside from these bosses, however, no known hitboxes in the game naturally have such angles, and there are no known hitboxes that hit opponents at an angle lower than what would be considered a semi-spike, but above what the game would consider a meteor smash. As such, spikes are considered a removed element in Brawl.