Armor (spelled armour in the PAL version of SSB4), also called flinch resistance, knockback resistance, super armor (for infinite knockback resistance), is an effect of certain moves or actions in all Super Smash Bros. games.
Armor is the concept of sustaining hitlag and damage, but not sustaining knockback. Most instances of armor have full damage taken; however there are exceptions. Some instances of armor, such as Piranha Plant's Long-Stem Strike or Mega Charizard X, can reduce what is taken after knockback calculation. Grab hitboxes ignore all variations of armor.
Types of Armor
When a character has super armor, they will generally not flinch nor take any knockback when attacked by a harmful hitbox. All bosses except Giga Bowser in Melee and Master Core's doppelganger form in Smash 4 cannot flinch or be knocked back at all, similar to the effects of super armor. However, they also cannot be grabbed.
Super armor has been given to a variety of moves with no true definition in terms of consistency. It has been given to some fast attacks and even grabs, such as Charizard's Fly and Little Mac's KO Uppercut, respectively. Many Final Smash transformations, such as Mega Lucario or Giga Bowser), also grant passive super armor. Some attacks may also give an opponent super armor, like Mario's Cape in Brawl and Smash 4, and Ganondorf's Flame Choke in Brawl.
Smash 4 contained numerous Custom Specials with Super Armor as alternatives to the default ones. Some of these were extremely quick or lasted long lengths of time, such as Volatile Breathing and Stubborn Headbutt. Custom Specials also allowed characters such as Toon Link to access armor when they otherwise wouldn't, in this case with Flying Spin Attack.
Most moves that use super armor are slow to offset the power of the mechanic, making them easily avoided. However, it isn't uncommon for them to deal significant shield damage as well, such as Little Mac's low angled forward smash. Some can also outright break shields, such as the Warlock Punch. Exceptions to this rule tend to be extremely unsafe on shield, with the aforementioned Fly being particularly unsafe, although this isn't mutually exclusive (Little Mac's forward smashes are all very unsafe).
Damage-based Armor was introduced in Brawl, specifically for Snake's Cypher. Under this effect, characters will suffer hitlag and damage, but not flinch or take knockback unless the attack deals enough percentage to go over a defined threshold. For example, Byleth's Aymr in Ultimate will handle up to 30% from frame 34 onwards before it can be interrupted.
This mechanic is usually applied to attacks used by super-heavyweight characters such as Incineroar to make up for their slow but powerful attacks. Most of the time, moves with this mechanic have low percentage thresholds for breakage, usually not going above 8%. Naturally there are exceptions to both of these rules, with characters such as the aforementioned Byleth having the mechanic applied to generally powerful attacks instead, even with high thresholds.
Ryu and Ken have a unique form of damage-based armor which accumulates its threshold every frame. It also lasts for only a single hit, allowing moves with multiple hits to break it. As of Ultimate, their thresholds begin at 14% and rise at 0.24% per frame, before finally capping out at 28%. Ken's armor frames also end a frame later than Ryu's.
This mechanic isn't without its weaknesses, however. For one, some moves such as Bowser's tilt attacks have very low thresholds, making the armor completely useless in high-damage matchups. Moves with the armor are also typically very unsafe on shield or whiff, meaning they can't be used without a plan and/or spacing. The attacks also tend to have low active armor frames in comparison to others, so they cannot be used for survivability very often. All the usual weaknesses of armor also apply, with moves such as Crownerang easily having the armor ignored by grabs due to the slow nature of the attack.
In Super Smash Bros. Ultimate, damage-based armor was distributed to a wide variety of characters. It also introduced the 1v1 multiplier, which causes characters to take more damage from attacks in one-on-one matches. In general, this does not alter whether an attack can break a move's armor: the extra damage taken is applied after all other calculations, including deciding whether the armor breaks. However, in the case of hitting an armored opponent more than once (or with a multi-hit attack), the higher damage dealt by previous hits may alter whether the next hit will succeed. For example:
This mechanic isn't to be confused with Belly Super Armor, a similar mechanic where King K. Rool's belly will protect him from attacks. In addition, while Kirby's Stone is damage-based, it isn't quite the same. It takes a cumulative amount of damage to break thereby making it more HP-based than damage-based. Furthermore, Kirby does not sustain damage until a move deals enough of said damage to actually deplete Stone's HP to 0 or less. As such, its status is disputed.
Knockback-based Armor was introduced in 64 as a fighter ability for Yoshi's double jump. This mechanic is used to prevent opponents from being knocked back unless it reaches a certain threshold in units. Some forms subtract the amount from the knockback received during calculation (Known as Subtractive Knockback-based Armor), such as the aforementioned Yoshi's double jump. Others, however, follow Damage-based armor's system, not sustaining knockback until the threshold is surpassed. This tends to be given as a passive fighter ability to characters rather than attached to attacks, however there are exceptions such as the Wario Bike. Crouch cancelling also gives a form of knockback resistance, although in Smash 4 and Ultimate, this has changed to be a 0.85* knockback multiplier.
The way knockback-based armor works varies by game. For some games it will subtract the knockback, making the difference between the threshold and knockback received what is taken. This was covered on the Japanese Super Smash Bros. website, covering Yoshi's double jump in 64. For certain other games, however, it will instead have characters take full knockback when the threshold is surpassed. In these cases, Knockback-based armor is separated by prefixes Subtractive and Non-Subtractive.
In Melee, the Ice Climbers - specifically Nana - have used knockback-based Armor as a passive ability. Nana has 5 units of launch resistance at all times, non-subtractive. Kyu Puff's tweet on the subject shows Nana not flinching when being hit by the weak hit of Luigi's up special. However, this has been removed in later games. Since Melee, Donkey Kong and Bowser have received mechanics similar to Nana's passive knockback-based armor for their Kong Karry and Tough Guy fighter abilities respectively.
The Metal Box offers knockback-based armor in that it gives users 30 units of Subtractive Knockback-based Armor. Combined with the increased weight, the fighter will never flinch if the total knockback taken is less than or equal to 0, which can be a substantially long length of time. However, the knockback resistance is ignored if the fighter is hit by an attack that has the bury effect like most forms of armor.
Knockback-based Armor is polarizing in terms of viability. Being generally quite low in numbers - only really notable for jabs - it rarely sees relevance during gameplay. However, moves such as Power Pellet and Yoshi's double jump are notoriously hard to break due to their extremely high thresholds, often being able to take smash attacks at what would otherwise be a KO percentage. For example, it takes over 70% from Samus' Charge Shot at full charge to break Yoshi's double jump armor.
In Ultimate, given that knockback is calculated before the 1v1 multiplier is applied to damage, knockback-based armor is unaffected by it. However, as in previous games, moves like Hero's Oomph and Psyche Up that do apply their modifications before knockback will affect it. Stale-move negation can also affect this through the freshness and staleness multipliers.
Faux Super Armor (Brawl)
Faux Super Armor occurs when two characters are hit with the same hitbox while one has the other in a grab. Whoever has higher controller port priority takes full damage but only flinching slightly from the attack, while the other takes full damage and full knockback.
This is often utilized for the Omnigay, a doubles combo popularized by Omni. This consists of having Meta Knight or Kirby use their up throw to send opponents close (or past) the blast zone, before a partner Snake blows up C4 attached to either character. This leaves the thrower unharmed, and the victim instantly KO'd. Due to the introduction of throw invincibility and more characters having throws akin to the ones used in the strategy, this has technically been buffed.
Grab Armor (Brawl)
In Brawl, when a grab connects with a character that also has an active hitbox, the character that attempted the grab takes the full damage from the hitbox but not the knockback, giving the illusion that grabs have a form of armor. However, this will only occur if the grab actually connects with the other character's hurtbox, and in both previous games, the same character would not have taken the damage.
In Smash 4 and Ultimate, this situation is reversed; the character that attempted the grab will now take full damage and knockback from the hitbox, and the other character will simply be grab released and take 3% damage (or more if the grab came from a special move and the grab itself dealt damage, like the beginning of Falcon Dive). In Ultimate specifically, a clash between grabs will also have characters take this damage, with an appearance similar to that of throw teching in traditional fighting games.