A spike is an attack that sends the opponent downwards but cannot have its knockback interrupted via meteor cancelling. As a result, they are significantly more dangerous than the similar meteor smash.
In most games of the Super Smash Bros. series, there are no spikes - all attacks that hit downwards are meteor smashes. However, in Super Smash Bros. Melee, a technical oversight results in two types of downwards attacks - those that can be meteor cancelled, and those that cannot - the first group are meteor smashes, while the second group are spikes. Despite this clear distinction, many players use the term "spike" to mean "meteor smash" regardless of the game, referring to spikes as "true spikes".
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 usable 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.