A combo is a bit more flexible in Smash than in many fighting games, because encounters are so situational, and defense is never disengaged. Broadly, a combo is a sequence of moves that hit a foe in a row for good damage, maybe even as a KO setup. But so many variables—widely varying fighter weights and matchups, different stage sizes, position onstage, increased knockback as damage percentages pile up, the ability of defenders to shuffle or use directional influence to steer—make it hard to speak generally about combos.
Often, players will specify they’re talking about a “true” or “guaranteed” combo when they mean an inescapable sequence, one that would increase the combo meter on the Training Stage.
Much of the anti-air/combo/juggle game blurs together, especially midstage. Many moves pop enemies into jugglable positions, but not for guaranteed combos: technically, as you rise to meet them with your up air attack (for example), perhaps they emerge from the controllable portion of their tumble with an earlier air move that beats yours, or perhaps they air-dodge through the juggle attempt and gain advantage, maybe even a guaranteed punish against you. So then it becomes more of a mind game: launching moves are still great, but instead of going for the obvious combo follow-up hit that’s not quite guaranteed, it’s a good idea to pretend to do that, then fast-fall and block, or air-dodge with downward momentum. With this fake aggression, tumbling defenders can be coerced into pre-emptive attacks or air dodges, which you can then punish, perhaps putting them right back into a similar situation.