A combo is a sequence of attacks that is guaranteed (or very likely) to occur if the first attack hits the enemy. The exact definition of a combo may change from player to player. Some definitions may include:
Combos are used to quickly rack up damage on opponents. In Super Smash Bros., long, highly-damaging true combos are commonplace, due to the hitstun being much higher than in subsequent games. The lack of DI contributes to this, resulting in attacks sending opponents in consistent directions. This is also compounded by the lack of wall- and ceiling-teching, which allows for well-known tent combos. In fact, every character in SSB64 has a confirmed zero-death combo.
In Super Smash Bros. Melee, hitstun was reduced, and the introduction to DI makes combos more difficult to pull off consistently. However, with the increased game speed and new mobility options, such as wavedashing, along with the ability to fastfall during an aerial attack, combos are still very common, usually racking up around 50% damage. The decreased knockback on throws and the aforementioned physics changes make chain-grabs more common. In both these games, L-cancelling also allows for faster attack speed, contributing to the ease of combos.
In Super Smash Bros. Brawl, characters can air dodge almost immediately out of hitstun, greatly reducing the frequency of combos. This is compounded by the slower falling speeds and more limited mobility, as L-cancelling and wavedashing were both removed. As such, true combos in Brawl tend to work only at very low percentages with quick moves that deal very low base knockback. The ability to negate hitstun results in gameplay having a heavy emphasis on locks, infinites and chain-grabs.
In Super Smash Bros. 4, the ability to air dodge out of hitstun has been removed, making true combos much more common, and the increased falling speeds also help in this regard. However, with the ability to air dodge out of a tumble, the increased base knockback on many moves, and the heavier landing lag on aerial attacks, true combos are still harder to pull off than in Melee and Smash 64, and are generally not as long nor as varied as in the aforementioned games. Characters can also no longer be locked indefinitely, and chain-grabs have been completely removed. As a result, SSB4 combos are often initiated with throws and often last for 2-3 hits, usually racking up 15-20% damage, with any further follow-ups requiring the player to read the opponent's defensive options.