Hitlag (also known as hitstop or freeze frames, and officially known as hitstun) is a phenomenon that appears in all games of the Super Smash Bros. whereby a character will freeze for a period of time after connecting with an attack.
If successful with most attacks, both the attacker and target are frozen in time for a number of frames. This causes the attack to get "stuck" out longer than it would if it had not hit anything, while the target has a short time to react before the knockback occurs. This is most easily noticed in multi-hit attacks such as Yoshi's down aerial; the attack takes less time to finish if it does not hit anything.
Once the hitlag has passed, both sides resume action. During hitlag, defending characters are capable of performing smash directional influence to get out of combos or multi-hit moves, or to increase their chance for survival.
In several other fighting games, the amount of hitlag frames often dictate the attack level of connecting attacks, ranging from light-to-heavy; the more frames there are, the heavier the attack in question is (factors often completely independent of damage inflicted). Hitlag often times allows for a window to transition into another attack via cancel if there exists such a window, or at least plan the next action both in the attacker's case (though as aforementioned solely in the case Smash, smash DI is an action the victim can utilize).
In visual cases however, an attack having high hitlag frames also sells its impact whenever it connects.
The formula for calculating the number of frames of hitlag experienced by both the attacker and victim has been different for most of the games:
For all these cases, the final result is rounded down. d is the amount of damage an attack would deal rounded down, while m is a series of multipliers based on certain factors, including:
As shown by the formulas, the general amount of hitlag on moves is at its lowest in Melee, and has since increased throughout the series, with Ultimate having the most hitlag. In all games, hitlag is higher the more damage a move deals. For example, weak attacks such as Mario's jab have minimal hitlag, but the hitlag of strong or sweetspoted attacks can last as long as half a second. In addition, Brawl introduced the mechanic of hitlag modifiers, causing variance in the hitlag duration of attacks; this is in contrast to Melee and Smash 64, where the duration of hitlag was predictable.
Hitlag only affects the object that deals the damage; all other game elements (including, interestingly enough, any particle effects the attack generated) are uninterrupted. For example, both Captain Falcon and his opponent sustain hitlag upon a sweetspotted Knee Smash, while Samus' movement is not interrupted by a Charge Shot hitting someone, since it is a projectile not attached to her. Hitlag affects the attacker as long as the attack connects, even if it deals no damage as a result of hitting opponents with invincibility. Hitlag is also exaggerated if two attacks clash, or if an attack is perfect shielded; in the latter case, the attacker suffers from hitlag while the defender receives none. Additionally, if an attack deals no knockback, the target does not experience any hitlag.
Attacks with the electric effect are unique in that they increase the amount of hitlag, multiplying the number of frames by 1.5 (rounded down), which stacks with the hitlag multiplier that the move otherwise has (for example, an electric attack with a hitlag multiplier of 1.2 deals 1.8 times the amount of hitlag). Interestingly, in SSB4 if a character is hit by an electric attack from another, and either character is affected by slowdown (such as the Timer item), then the target receives additional hitlag, while the attacker does not. This phenomenon does not occur with non-electric attacks, even those that have a hitlag multiplier.