Armor

Ike's super armor from his Aether move. Notice how Ike didn't take any knockback from Critical Hit, which is an invariable OHKO under standard circumstances.

Armor, also called launch resistance, knockback resistance, super armor, or heavy armor (when the knockback resistance is finite), is a phenomenon that manifests itself during certain actions in all four Super Smash Bros. games. It is defined as a character not flinching at all, or in some cases taking less knockback than usual. Note that armor only affects knockback. The attack's full damage will still be taken. Most cases of armor are moves with an infinite amount of knockback resistance, and so are often referred to as super armor. Cases of finite amounts of armor are commonly referred to as heavy armor (or even light armor if it only works against extremely weak attacks, such as jabs or individual hits of multi-hitting attacks).

Faux super armor occurs when two characters are hit with the same hitbox while one has the other in a grab. When this occurs, 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.

In Super Smash Bros., Melee, and Brawl, when a grab connects with a character that also has an active hitbox, the character that attempted the grab will override the hitbox completely and not take any knockback. In Brawl, the same character also takes the damage from the hitbox, giving the illusion of "grab armor", though it is not to be confused with true launch resistance, as this will only occur if the grab actually connects with the other character's hurtbox. In Smash 4, 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 only flinch slightly and take 3% damage or more, depending on the type of grab that was used.

Note that grab hitboxes ignore all variations of armor, and many bosses (except Giga Bowser in Melee and Master Core's final form in Smash 4) cannot flinch or be knocked back.

In Super Smash Bros. and Super Smash Bros. Melee

Yoshi double jumping through an attack.

Armor is a very uncommon occurrence in the first two games, being limited to Yoshi's double jump cancel counter and crouch canceling, which will reduce knockback to 2/3 of the original value in Smash 64, and 1/2 in Melee (at the cost of poor survival directional influence). The difficulty setting on the single player modes (and events in Melee) also affects the ability to be launched for both the player's character and the CPU characters, with the player's character taking more knockback and the CPUs taking less knockback as the difficulty setting increases. Adjusting the handicap levels in the multiplayer modes in Melee has the same effect.

According to the Japanese Super Smash Bros. website, Yoshi's double jump resistance works by subtracting 120 from the knockback that would have been delivered, with Yoshi not flinching if the result is negative, and Yoshi taking the difference in knockback if the result is positive.

In Super Smash Bros. Brawl

"Super armor" was introduced in Brawl; several characters have moves that provide them with infinite launch resistance, usually to allow for such characters to counter attempts to disrupt the incoming attack. Some of these moves are not offensive, however, and there are a few cases of finite launch resistance, as listed below. Crouch canceling no longer affects knockback, but instead affects the amount of freeze frames sustained.

All of the moves are super armor unless otherwise stated.

Character	Moves	Conditions and Notes
Bowser	Giga Bowser	Giga Bowser: For the entire period of being Giga Bowser.
Diddy Kong	Rocketbarrel Boost	Only the grounded version (this is a variation of heavy armor).
Donkey Kong	Giant Punch, Spinning Kong and Carry Throw	Giant Punch: only if fully charged; while moving his arm forward but the attack needs to make contact with his left arm and before it is released. Spinning Kong: only the grounded version; just before he starts to release it (frames 10-16 first hitting on frame 19); if he leaves the ground with Super Armor frames when using the grounded version, a glitch will make the Super Armor frames last for the entire move. Carry: DK gains knockback-based launch resistance for the entire duration of the Carry. Also works while holding heavy items.
Ganondorf	Flame Choke	In the grounded version, when holding the opponent off the ground; the opponent also gets super armor. In the aerial version, the opponent gets super armor but Ganondorf does not, allowing for strange KOs in places like Norfair or Brinstar where Ganondorf will bounce off the acid, while the opponent will continue their plummet.
Ike	Aether and Eruption	Aether: After throwing his sword in the air, but before he leaps up towards his sword. Eruption: Some frames after the special move button is released but before the sword hits the ground.
King Dedede	Super Dedede Jump	From the start until just before the apex of the jump.
Kirby	Stone	The entire duration of the period Kirby is in his Stone transformation, where he will take no damage or knockback, but he can only take up to 29% damage per Stone before he actually sustains knockback. If Kirby takes 30% (or more) damage during a single Stone transformation, he'll be knocked out of Stone and will take the full knockback -- but strangely none of the damage -- of that hitbox (this is a variation of heavy armor).
Olimar	Pikmin Order	During the first few frames. Lasts until right after he puts his head down.
Pit	Mirror Shield	Grounded version only; while pulling the shield out: frames 4-6.
Pokémon Trainer	Fly and Squirtle's forward smash	Fly: The beginning of the ascension times: Frames 4-13, making it useful when jumping out of shield and an effective combo breaker. Squirtle's forward smash: While moving forward.
Snake	Cypher	Snake cannot be flinched by any hitbox that deals exactly 7% or less, regardless of the knockback of the individual hitboxes. Most multi-hit moves deal less than 7% per hit, so they typically cannot interrupt the Cypher (a variation of heavy armor). (See this post and this followup post on SmashBoards for more details.)
Wario	Forward Smash and Wario Waft	Forward Smash: While moving forward: frames 8-12 Wario Waft: When fully charged, during the first few frames.
Yoshi	Double jump	Yoshi gains (approximately) 3860mph-worth of knockback resistance, as measured by the post-Brawl stats (under the "top launch speed"). This means that Yoshi will ignore any knockback that would launch him at less than ~3860mph while in his midair jump. As he is damaged, he takes more knockback from an attack with the same launching power, meaning that weaker attacks will begin interrupting his midair jump as Yoshi approaches higher percentages (a variation of heavy armor). (See this post on SmashBoards for more details.)

In Super Smash Bros. 4

Smash 4 continues the trend of implementing launch resistance to certain characters, though there is more variation with the strength of armor (some negligible, some infinite, and some based on damage). Crouch canceling now acts as a combination of Melee and Brawl's version of the tactic, although the knockback sustained is only reduced to 6/7 of the original value.

All of the moves are super armor unless otherwise stated.

Character	Moves	Conditions and Notes
Bowser	All animations and Giga Bowser	Bowser has universal armor against attacks that deal negligible knockback. Giga Bowser retains the infinite knockback resistance from Brawl.
Bowser Jr.	Clown Kart Dash	This move has a resistance to moves with negligible knockback, the same as Bowser's universal armor.
Charizard	Rock Smash, Rock Hurl, Fly, Rising Cyclone, Flare Blitz, Blast Burn	Rock Smash: During the move's start-up, starting from frame 5. Is negated if the boulder is broken before Charizard headbutts it. Rock Hurl: Same as Rock Smash, except the super armor starts on frame 1. Fly/Rising Cyclone: Just like in Brawl, it has super armor during the move's start-up. Flare Blitz/Blast Burn: During the active charge portion of the move, it has heavy armor, where it'll armor through hitboxes that don't deal more than 14% damage.
Dark Pit	Electroshock Arm	During the uppercut.
Donkey Kong	Giant Punch, Spinning Kong, Stubborn Headbutt and Kong Cyclone	Giant Punch: Only receives full super armor when the move is fully charged, when his arm is moving forward just before and during the punch (on frames 11-20). Spinning Kong: Only the grounded version, DK gains super armor when he pulls back or just before he starts spinning (frames 8-17, first hit on frame 19). Stubborn Headbutt: Gains super armor for the entire move, including start-up. Kong Cyclone: grounded version - from the 2nd hit in front until shortly before the last 2-3 swings; the aerial version seems to be the same case
Ganondorf	Warlock Punch, Warlock Blade, and Dark Fists	Warlock Punch / Warlock Blade: When initiated on the ground, the move will have super armor until Ganondorf begins his punch/thrust forward. Does not have super armor if initiated while Ganon is airborne, even if Ganondorf lands on the ground before punching. Dark Fists: During its start-up, from frame 6 to 16 (when the hitbox of the first punch comes out).
Ike	Eruption, Aether	Eruption: Only 3/4 to fully charged: shortly before the release (ends before hitbox appears) (frame 6-10). Aether: while his sword is thrown upwards
King Dedede	Super Dedede Jump, Rising Dedede, Armored Jet Hammer	Super Dedede Jump/Rising Dedede: Just like in Brawl, it has super armor during the rising portion of the move, until Dedede reaches the apex of his jump. Armored Jet Hammer: Once Dedede starts charging the move, it has super armor until Dedede unleashes the move or is interrupted by other means.
Kirby	Stone, Grounding Stone, Meteor Stone, Hammer, Giant Hammer	Stone/Grounding Stone/Meteor Stone: Kirby can sustain up to 24% damage while transformed, where he'll take no knockback from all moves regardless of how strong their knockback is, while also taking no damage. Once he sustains more than 24% damage while transformed, he'll be launched as normal, while taking half the damage from the threshold-breaking hitbox. Hammer: When fully charged, it'll have super armor during its swing. Giant Hammer: Once Kirby begins charging the move, he'll have super armor until the frame he unleashes it or until he is interrupted by other means.
Little Mac	Straight Lunge, all smash attacks, and Giga Mac	Straight Lunge: During the charging and execution. (damage based) Smashes: During their start-up. Giga Mac: The entire duration of the Final Smash.
Mewtwo	Shadow Ball Custom (Glitch)	After fully charged, when Mewtwo freezes in place.
Mii Brawler	Ultimate Uppercut and Exploding Side Kick	Ultimate Uppercut: During the start-up of the punch, only if fully charged. Explosive Side Kick: During the start-up, before kicking.
Olimar	Pikmin Order	Pikmin Order's armor frames are lesser than it was in Brawl.
Pac-Man	Power Pellet	Towards its end.
Pit	Upperdash Arm	During the uppercut.
Roy	Blazer	Grounded version during startup.
Ryu	Focus Attack	While charging the attack, Ryu will armor through a single hitbox of any strength, but will be hit as normal if struck by a second hitbox regardless of damage/knockback.
Yoshi	Double jump	Yoshi retains the knockback-based heavy armor from Brawl.
All characters	Home-Run Bat and Ore Club	Home-Run Bat: When the bat is about to be swung. Ore Club: When charging and releasing a smash attack.

External references

• A better 'Super Armor List' Thread: lists characters and their moves that have Super Armor, along with some frame data.
• The Armor Project: explains how knockback-resisting moves that are not super armor work, including Snake's Cypher and Yoshi's double jump.