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.

Launch resistance, also called knockback resistance, super armor, or heavy armor (when the knockback resistance is finite), is a phenomenon that manifests itself during certain moves in all four Super Smash Bros. games. It is defined as a character sustaining less knockback than usual when hit, or in most cases not flinching at all. Note that knockback resistance, as the name implies, only affects knockback. The attack's full damage will still be taken. Yoshi's double jump, Snake's Cypher, Diddy Kong's Rocketbarrel Boost (on the ground), and Kirby's Stone are the only moves with finite amounts of knockback resistance, and are commonly referred to as having heavy armor. All other knockback resistance moves have infinite knockback resistance and are commonly referred to as super armor.

Note that grab hitboxes ignore all variations of launch resistance.

In Super Smash Bros.

Yoshi double jumping through an attack.

Knockback resistance was limited to Yoshi's double jump cancel counter, most characters while throwing, Giant DK at low percentages, and Metal Mario. Master Hand has infinite knockback resistance.

While most things in the above list have unknown amounts of resistance, it is defined on the Japanese Super Smash Bros. website that 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.

In Super Smash Bros. Melee

Yoshi retained his resistance, while Metal and Giant characters had their resistance drastically decreased. Giga Bowser also has some amount of knockback resistance when fought in Adventure Mode. Master Hand and Crazy Hand both have infinite knockback resistance.

In Super Smash Bros. Brawl

Many characters have knockback resistance, usually in the form of moves that give a temporarily infinite amount (a.k.a. super armor). All of the moves are "super armor" unless otherwise stated. Also, it is possible to set the character's launch resistance.

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.)
All characters	All grabs except Olimar's grab. This includes special moves that operate as a grab such as Chomp, Egg Lay, Inhale, Super Inhale, Falcon Dive, Dark Dive, Flame Choke, Force Palm, Flying Slam.	In Brawl, when a character successfully grabs an opponent while simultaneously connecting with a harmful hitbox, the grabbing character will have super armor and take no knockback or hitstun from the hitbox. Another form of launch resistance occurs when both 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 no knockback at all, while the other takes full damage and full knockback. This phenomenon is known as faux super armor, and unlike standard grab super armor, this variation does not occur with special move grabs.

In Super Smash Bros. 4

Smash 4 possesses all the super armor moves from Brawl, except for the moves that were removed and "grab armor", causing them to be interrupted by lingering hitboxes. Several abilities have also received armor.

Character	Moves	Conditions and Notes
Bowser	All moves	Bowser has universal armor against attacks that deal negligible knockback.
Bowser Jr.	Clown Kart Dash	?
Charizard	Rock Smash	During the move's startup. Is negated if the boulder is broken before Charizard headbutts it.
Dark Pit	Electroshock Arm	During the uppercut.
Donkey Kong	Stubborn Headbutt Giant Punch Spinning Kong Carry Throw|| Giant Punch: Only when the move is fully charged and receives super armor just before and during the punch. Spinning Kong: Gains super armor on the grounded version only when he pulls back before he actually starts spinning. Stubborn Headbutt: Gains super armor for the entire move, including startup. Carry Throw: Gains super armor for 1 second just when DK picks opponent up.
Ganondorf	Warlock Punch and Dark Fists	Warlock Punch: During the grounded, non-reversed version while charging up, before punching. Dark Fists: During its start-up.
Little Mac	Straight Lunge and all smash attacks	During the charging of Straight Lunge and the startup of all of Mac's smashes.
Olimar	Pikmin Order	Pikmin Order's armor frames are lesser than it was in Brawl.
Mii Brawler	Ultimate Uppercut and Exploding Side Kick	During the startup of the punch, if fully charged. The Explosive Side Kick has super armor similar to that of Ganondorf's Warlock Punch.
Pac-Man	Power Pellet	Towards its end.
Pit	Upperdash Arm	During the uppercut.
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.