Lag is a catch-all term used to indicate periods of time where a character is busy initiating or finishing a performed move, being left unable to perform any other action (except for moving through the air with aerial moves). The two main categories of lag that every move in the game has are startup lag and ending lag, and for aerials and certain special moves when used in the air, there's an additional form of ending lag that can occur called landing lag. The more lag a move has, the easier it is to evade the move and punish the user. Typically, powerful moves have more startup and/or endlag, though this is not always the case. Lag is usually measured in frames, with one frame being the same length as 1/60th of a second (or 0.0166666667 seconds).
Start-up lag, also known as just start-up and windup, is the delay between a move being initiated and the move having an effect, such as the length of time before a hitbox is first produced. The fastest any move can come out is on frame 1 (1/60th of a second) and the more frames it takes for a move to come out, the longer its startup lag will be. Examples of moves with extreme start-up lag include Ganondorf's Warlock Punch and King Dedede's forward smash. Examples of moves with no start-up lag include Fox's and Falco's Reflector in Melee and Jigglypuff's Rest. Flinching during start-up will prevent the move from being executed, unless the move grants the character invincibility/intangibility or armor.
The primary advantage of lower start-up lag is that the attack gives the opponent less reaction time, as well as the ability to hit before other attacks, giving the user greater flexibility in being able to land the attack (making such attacks all-around easier to land). Moves with lower startup lag can be used in a greater amount of situations, such as being a pressure escaping tool or as a punishment option. Moves with higher startup lag however cannot be used in nearly as many situations as the opponent can usually punish the attacker for simply throwing out the move before it can connect. Moves with very high startup lag are rarely used as a result, only being useful as a risky hard punish or as a shield break punish in most situations. While not always true, characters with fast frame data attacks tend to be more successful in competitive play then those with slow frame data attacks; part of the reason why characters such as Pikachu, Fox, Sheik, and Meta Knight typically rank in the top or high tiers in Smash tier lists is that the general start-up lag of their attacks are among the fastest. Conversely, a major part of why characters such as Ganondorf are typically ranked as low tier characters is that the general start-up lag of their attacks are among the slowest.
Ending lag, also known as cool down and recovery in other fighting games, is the delay between a move's effect finishing and another action being available to begin, such as the length of time after an attack's hitboxes ceases before the character can move again. Almost all attacks have more ending lag than startup lag, though generally, attacks with lower start-up lag tend to have proportionally more ending lag and vice versa. Attacks such as Rest and Ike's forward smash are notorious for having an extreme amount of ending lag. Moves known for having especially low ending lag include Meta Knight's up aerial in Brawl and Lucario's forward smash. Typically, an attack's ending lag can't be avoided, but some attacks can avoid ending lag without the character being flinched or out-prioritised, such as grabbing a ledge with a recovery move before it ends, and jump canceling Fox's and Falco's Reflector in Melee. Additionally, aerials can avoid their ending lag by the character landing before completion; despite this inducing landing lag, it can be overall faster if the character lands soon enough and the attack's landing lag is low enough (a prominent example of this is Ike's neutral aerial, which has a huge amount of ending lag, but rather low landing lag, thus landing earlier can be utilised to circumvent the attack's high ending lag). Also, if a character using a standard land attack is no longer on land before the attack finishes (such as from being pushed off by wind or the platform underneath them ceasing to exist), the attack will abruptly end with the user free to move, thus avoiding the attack's ending lag (though special moves will continue if this occurs while using them).
The primary advantage of lower ending lag is that the less ending lag there is, the less punishable the move is, thus the more safe it is to use. Since the attacks end sooner, low ending lag attacks are also more effective at comboing, as they allow a greater window to follow-up landed attacks before the opponent can respond properly or respond at all. An example of this in Smash 64 can be seen with Kirby and Donkey Kong's up tilts. Both up tilts have the same startup lag but Kirby's has much less ending lag, which makes it drastically harder to punish as well as making it a much more effective combo tool compared to Donkey Kong's. While having attacks with low ending lag is considered important in competitive play and characters higher up on the tier list typically have lower ending lag attacks, it isn't held in equal regard to start-up lag. Marth, for example, is considered a top/high tier character in all of his iterations, despite most of his attacks having high ending lag, while there are no characters in any of the games ranked high that have comparatively high start-up lag on their attacks. More attacks can be very effective in spite of high ending lag (Jigglypuff's Rest in Melee being the most notorious example of such, as despite its ridiculously high ending lag and small hitbox, its start-up lag of 1 frame makes the move very landable, where its extreme power can be utilised), while there are few attacks that can be very effective in spite of high start-up lag. For example, Ganondorf's up tilt in Melee and Brawl is an extreme power attack with a small hitbox that has proportionally low ending lag, but its start-up lag is so ridiculously high that the move is almost impossible to land without the opponent being incapacitated, in a position where they have no choice but to position themselves within its reach (such as attempting to recover or evade a stage hazard), or having already committed to an action that will put them in the attack's reach (i.e. Ganondorf successfully reading his opponent), and its extreme power can realistically never be utilised (however, in Smash 4, it has a significantly larger hitbox). Moves that are very effective in spite of high start-up lag have a combination of significant attributes beyond just low ending lag (the aforementioned forward smash of Lucario's is considered to be one of the best forward smashes in Brawl despite being one of the slowest in start-up, as the move not only has very low ending lag, but also has potential extreme power with great reach, making it one of the few powerful KO moves that can often be used with no repercussions if it fails to land).
Landing lag is the lag incurred when an airborne character lands on the ground for any reason. The amount of lag depends on what action the character is performing upon landing.
When landing under normal circumstances, characters undergo either a "light landing" (also known as a "no-impact land") or a "heavy landing". In Melee, there was only one type of normal landing animation so most characters have a 4 frame landing animation, regardless of how fast they are falling (which a few exceptions). Light landings incur only 2 frames of lag (4 frames in Smash 64), whereas heavy landings are character-specific (after Smash 64 where all heavy landings had 8 frames of landing lag), ranging from 4 to 6. The type of landing that occurs is based on the character's downwards speed upon landing, so landing during the apex of a jump or on an upwards-moving platform tends to be faster than landing from a fastfall or on a downwards-moving platform. Brawl's air dodges always have normal landing, regardless of the character's height prior to using them.
If a character lands while in the middle of an aerial attack, they go through a unique animation that lasts for significantly longer than a normal landing. This animation is not automatically determined. The game has to call for a flag which starts on a pre determined frame in order for the aerial landing lag animation to occur. Any time a character lands while this flag is not activated, they will enter their normal landing animation (this is known as an auto-cancel). A clear example of an aerial landing animation can be seen with Link's down aerial; hitting the ground during the attack results in Link's sword getting stuck, and he has to spend a significant amount of time pulling it out before he can do anything else (especially prior to Smash 4). Each aerial attack has its own amount of landing lag. Naturally, the less landing lag, the better; for the same reasons as why less ending lag is beneficial. Most special moves tend to continue execution instead of being interrupted when the user lands while using them, though a few undergo their own unique landing animation if the character lands after initiating them in the air, such as Falcon Kick and Wizard's Foot and some such as Fox's lasers prior to Smash 4 will even make the character enter their normal landing animation.
Certain aerial attacks produce a hitbox during their landing lag, such as Kirby's down aerial.
In Smash 64, not all aerial attacks have unique landing animations, though they still have unique lag durations.
There are ways to circumvent landing lag. In Smash 64 and Melee, a technique called L-cancelling (more commonly known as Z-cancelling in Smash 64) can be used to reduce landing lag. Pressing shield just before landing will reduce the amount of landing lag an aerial has. In Smash 64, this will make the character enter their normal landing animation although in Melee, it plays the same landing animation but only at double the speed (effectively halving the landing lag). Because of this, the technique is more effective in Smash 64 although it is still a vital technique in both games, making aerials safer when landing and increasing their followup potential. In all games, an aerial's landing lag can avoided by landing during the time they can auto-cancel. Each aerial has a different auto-cancel window and the more time an aerial can auto-cancel, the easier and more beneficial/practical it is to auto-cancel. Aerials with higher landing lag especially benefit from these techniques .
Smash 4's air dodges have their own aerial landing lag, being universally 21 frames for all characters. When landing one frame before the end of an aerial or air dodge's animation, aerial landings can be completely cancelled into special moves, such as after Ganondorf's forward aerial from the start of a full hop (as Ganondorf is in the air for 44 frames during his full hop and his forward aerial's first actionable frame is on frame 45). Ultimates air dodges have 10 frames of landing lag when a normal air dodge is performed and 19 frames when a directional air dodge is performed.
Characters that land while in their helpless state undergo a specific amount of landing lag, dependent on the move that initiated it. While the state is known as "LandingFallSpecial" or similar in the games' code, it also applies to air dodges, and thus wavedashes, in Melee (with air dodges having 10 frames of landing lag in Melee).
Bayonetta's landing lag gets higher depending on how many special moves she used before landing. This applies to both the normal landing lag and the landing lag from aerial attacks. These are known as her recovery frames.