Freeze frames (also known as hitlag or hitstop, and officially known as impact stall) are a phenomenon that appears in all games of the Super Smash Bros. series 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 freeze frames have passed, both sides resume action. During freeze frames, defending characters are capable of performing smash directional influence to get out of combos or multihit moves, or to increase their chance for survival.
The number of freeze frames (i.e. the length of the freeze) depends on the attack. Weak attacks such as Mario's jab have minimal freeze frames, but the freeze frames of strong or sweetspotted attacks can last as long as half a second. Freeze frames are also exaggerated if two attacks clash, or the attack is perfect shielded. In the latter case, the attacker suffers from hitlag while the defender receives none. If either side is under a time-slowing status, their freeze frames will be affected, but the other side's will not, causing the two to unfreeze at different times.
Freeze frames only affect the object that deals the damage. All other game elements (including, funnily enough, any particle effects the attack generated) are uninterrupted. For example, both Captain Falcon and his opponent will undergo freeze frames 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. Freeze frames affect the attacker as long as the attack connects, even if it deals no damage as a result of hitting opponents with invincibility.
Electric attacks have a special effect on freeze frame duration. The player being hit by the electric move will have the duration of their freeze frames multiplied by 1.5 (rounded down), while the attacker will not receive any additional freeze frames.
The general amount of freeze frames on moves has increased throughout the series, with Brawl and SSB4 having more freeze frames than Smash 64 and Melee. In addition, Brawl introduced the mechanic of hitlag modifiers, which are multiplicative modifiers attached to some hitboxes that change the duration of freeze frames caused by the attack, causing variance in freeze frame duration. This is in contrast to Melee and Smash 64, where the duration of freeze frames was predictable. In Melee, the formula for calculating freeze frames is d/3 + 3 rounded down, where d is the amount of damage the attack would deal, ignoring stale-move negation.
Hitlag is a mechanic that was borrowed from traditional fighting games.