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 erroneously use the term "spike" to mean "meteor smash" regardless of the game, referring to spikes as "true spikes" to reflect their inability to be cancelled.
In Melee, hitboxes that send opponents at an angle between 260 and 280 degrees are recognized 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 recognize them as meteor smashes, and are thus unable to be meteor cancelled.
It is most likely due to the degree/angle they are sent at being akin to the volleyball strategy of the same name (instead of directly downward), is possibly why they are named as "spikes"; spikes in volleyball are also unable to normally be returned, in the same way spikes in Smash Bros. cannot be meteor cancelled.
In Brawl however, the angle window for what the game recognizes 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 recognized 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 other games
In Brawl and Smash 4, several boss and enemy hitboxes have angles such as -70 and -90, which appear to be intentional ways of circumventing the meteor smash recognition window for the former game, making such attacks spikes. As playable characters do not possess any such attacks, however, spikes are considered a removed element in such games.
List of spikes by enemies and bosses in Brawl
List of spikes by enemies and bosses in SSB4
Note that at the present time, while most hitboxes are known, their associated moves may not be.