Combo

A combo is a sequence of attacks that do not allow the opponent to break out of hitstun. Combos are not necessarily guaranteed however — many factors play into their effectiveness, such as DI, SDI, teching, damage, the opponent's weight, and timing of the attacks.

History
Combos exist in every  game at some level, and are a crucial technique for both mid-level and high-level play. They are considered as a large factor when deciding a given character's placement on tier lists.

In Super Smash Flash 2
Creative combos were not possible in the  series until demo v0.5a, when the engine's knockback physics were retooled specifically to allow them. In Super Smash Flash 2, characters were not coded by developers with combos in mind until v0.7 (bar v0.5a's ). Prior to v0.5a, strings were very prominent and were actually referred to as combos by most of the community. All characters since the engine re-haul have been designed with the combo mechanics in mind, but some are designed to do so better or worse than others.