Combo

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:

Any sequence of attacks that keeps the opponent in hitstun
Any sequence of attacks that counts as a combo according to the combo counter (which allows for one consecutive frame of non-hitstun)
Any sequence of attacks that is very hard to DI (or in certain cases, tech) out of
Any sequence of attacks that cannot be DI'd out of if performed perfectly
Any sequence of attacks
A single attack that consists of multiple hits (see natural combo)
Any sequence of attacks that prevent the opponent from landing (possibly by tech) or ledge-grabbing
Any attack that sends an opponent into a hazard and then leaves them vunerable to other attacks.