Overhead attacks bypass a low block by attacking downwards from a grounded position, while standing overhead (hence the name). Overhead attacks can be blocked high, however, and this necessity to block an attack at the correct height creates a "guessing game", where a defender must attempt to guess where the next attack is coming from, and an attacker must guess which way the opponent will block.
Overhead attacks can come in the form of special attacks or unique attacks. Not every fighter character possesses an overhead unique attack, nor does every fighter possess an overhead special attack; however, most characters possess at least one or the other, and every character can also perform a jumping attack, which will also bypass a low block.
Street Fighter III seriesEdit
Characters in the Street Fighter III series also have a "universal" overhead, also known as a "leap attack". It is executed by pressing down twice and then pressing any punch or kick button simultaneously. In 3rd Strike, it is performed by pressing medium punch and medium kick simultaneously. These leap attacks can travel over many low-aiming moves, and are useful for poking an opponent out of "turtling", or defending constantly while ducking. Some characters use a preset normal attack for their animation (for example, Ryu uses his jumping medium punch).
This type of leaping overhead is also a staple for other characters in other fighting games; however, some usages of this type often vary in that they may possess more unique hitboxes and jump-arcs; some may not even be overheads at all. There are also very few other fighting games with a universal overhead for all characters in this scheme, only they may not involve a hop into an attack. Regardless, the most common theme of these kinds of hop attacks, overhead-or-not, is to crush/bypass low-hitting attacks of many kinds.