In general terms, a punish is any reaction that penalizes an opponent. However, this term is mostly used to describe a specific transaction: whenever a player extends an attack that leaves them vulnerable, going through end lag, and their opponent responds with a punishing attack, causing unavoidable damage. For instance, when a player’s laggy down smash whiffs or gets shielded, they can be punished by most reprisals as they’re stuck in the attack’s retraction animation.
Punishing is a core concept in fighting games. Almost all attacks are punishable in some form, although some are less practically punishable than others. (Since you can perfect-shield and remove all shield lag, you can theoretically punish all jabs with a quick one of your own, though in practice this is really unlikely unless you’re a DARPA AI project playing Super Smash Bros. Ultimate competitively.)
In this guide, “punish” usually refers to attacking an enemy who’s either whiffed an attack, or whose laggy attack you’ve just shielded. Whether an attack is punishable is determined by how much shield-stun it inflicts and how much end lag it has before the attacker can raise their own shield or dodge. Range is also super important: the farther away an attack is used, the less likely opponents can close the gap in time. If the only things they can punish you with quickly enough are a short-range neutral attack and shield grab, and you poke from outside that range, then you’re safe even if the frame situation technically says otherwise.
In Super Smash Bros. Ultimate, most blocked close-range melee attacks are vulnerable to quicker shield grabs (think Mario speed, not heavyweight speed) if the attacker is within range. Against an immediate shield grab, attackers can’t jump away or dodge in time even if they buffer those actions. However, retaliating after blocking isn’t so simple since there are many variables to consider, including whether the enemy will end up too far away, or even behind you, after the attack. High-level players will seek to end up at max range or at your back on purpose, negating the counterattack speed of shield grabs.
Some attacks are good for punishing foes in these positioning scenarios. For instance, Cloud’s up tilt can hit enemies behind him quickly, so it makes for a powerful anti-cross-up punish attack. Many fighters’ side tilts and down tilts are also effective retorts against foes just outside of jab. Many side and down tilts start relatively fast, usually with good horizontal reach. Of course, this becomes matchup-specific, so learning the ins and outs of your fighter’s moves (and the major places where common opposing fighters are vulnerable) is vital to your punish game.