Knockback is a measure of how far an attack sends its target. For example, Bowser's forward smash is an attack of high knockback; it launches opponents very far, so far that at high damages it's more powerful than a Home-Run Bat's smash attack. Pikachu's neutral A, on the other hand, has very low knockback - it hardly sends opponents anywhere, even at ludicrous damages.
The knockback of most attacks increases as the target's damage increases. Knockback is also affected by the target's weight, gravity, and a few other conditions (such as type effectiveness). The damage dealt by an attack is a significant factor in how much knockback it deals, but it is not the primary factor.
Each hitbox of a move has two knockback values: a base knockback and a knockback scaling. Moves with high base knockback deal high knockback under any circumstances, such as the swing of a Home-Run Bat. On the other hand, moves with high knockback scaling take less damage to reach KO potential, such as Luigi's forward smash.
Certain attacks have set knockback. These attacks will always deal the same amount of knockback, regardless of the target's damage. The most famous attack with set knockback is Fox's Reflector in Melee; other examples include Luigi's taunt, the first hit of Young Link's forward smash, and Falco's blaster. Set knockback remains affected by characters' weight and other such factors.
In Brawl, knockback is given in match results as a number measured from 0 (no knockback) to over 60,000 (Critical Hit at 300%). A knockback between 5,500 and 6,500 is where characters start to be KO'd. The unit is given as "mph"; while this would presumably mean "miles per hour", this is obviously not the case, as a 1,000mph hit barely sends characters anywhere. The measurement is actually reported once multiplied by 1000; a displayed knockback of 3400 is understood by the game as 3.4.
Other physics of knockback
- If a Meteor Smash is landed on a grounded opponent, they will be sent upwards with slightly reduced knockback (20% less in Brawl).
- If a character is hit while charging a smash attack, they will take slightly more knockback (20% more in Brawl).
- Based on type effectiveness, Pokémon Trainer's Pokémon will take more or less knockback from certain moves.
- At certain launch velocities, tumbling characters that impact another character may transfer some "momentum", dealing damage and knockback despite never being touched by the initial move.