"We've trained our bodies such that we can turn them into "iron carapaces" if we so choose..."
― Blueno describing Tekkai/Iron Body (One Piece)
"And she just doesn't get hurt! When she's wrecking the towns, sinking battleships or eats direct gun or cannon fire. Her tough as nails body simply doesn't suffer as much as a scratch! As you said, she really is an iron balloon!"
― Capone Luke "Gang" Bege on Charlotte Linlin/Big Mom (One Piece)
"You think a little heat is gonna bother me? I'm Lobo, I ride through the cold of space and the heat of blazing suns!!"
― Lobo after taking Superman's Heat Vision (DC Comics)
The user's body-tissue, skin, muscles, bones, etc. are extremely dense/solid, allowing them to withstand harmful forces on the body such as crushing weights, impacts, pressures, changes in temperature, etc. In addition, users usually possess drastically enhanced levels of strength and stamina, due to how durable their body has become, allowing them to resist tearing muscles and/or breaking their bones.
Enhanced Durability: User's durability is beyond peak human limits, but not to high superhuman levels.
Supernatural Durability: User's durability is greatly superhuman, being able to withstand extreme temperatures and pressures, virtually all corrosives, punctures, and concussions without sustaining injury.
Kenpachi Zaraki (Bleach) possesses tremendous durability, his immense spiritual pressure acting as an armor of sorts. Even Ichigo's hands got injured in a backlash without harming Zaraki.
Nnoitra Gilga (Bleach) using Hierro/Iron to pressurize his spiritual energy onto his skin, turning it iron hard.
Baraggan Lousenbairn's (Bleach) Resurrección grants him tremendously resilience, allowing him to survive Suì-Fēng's Jakuhō Raikōben's massive explosion at point blank range.
Ichigo Kurosaki (Bleach) using Blut Vene to flow spiritual energy into his bloodstream, turning it inhumanly hard.
Cang Du (Bleach) lives up to his epithet, "I - The Iron", turning his skin into impenetrable iron.
Gremmy Thoumeaux (Bleach) using "V - The Visionary" to imagine his body hardening to stronger-than-steel.
Ryūichi Sendō (Code:Breaker) transforming his skin into steel, increasing his defense and offense.
Skinn Bolic (D.Gray-Man) possesses an incredibly strong layer of skin composed of solidified lightning.
While in his Legendary Super Saiyan form, Broly (Dragon Ball Z: Broly - Second Coming) reinforces his body with tremendous ki to the point where he is completely unaffected by Super Saiyan Gohan's punch.
Tagoma (Dragon Ball Super) takes a punch to the face from Piccolo without even flinching.
Torafusa (Fairy Tail) can alter the density of his skin that can not only stop Gajeel's magical iron but break it as well.
The Captain (Hellsing)'s tissues are hard enough that he can catch Walter's Wires in his hands, even though they can cut through whole skyscrapers.
Tōkajin (InuYasha) implanted Shikon shards into his body, turning it into an armor of stone.
Unruly Fourth Raikage A/Ay (Naruto) inherited his father's near indestructible body, though a lower level in comparison.
Kakuzu (Naruto) using Earth Spear to transform his skin into a harder-than-steel material that is near impossible to penetrate, aside from Lightning.
The Third Raikage (Naruto) possesses a near indestructible body even without his Lightning Release Armor.
Trained in Rokushiki/Six Powers - Tekkai/Iron Body since childhood, Blueno (One Piece) can make his entire body as dense as an iron plating, causing Tilestone's hammer to shatter upon recoil impact.
Tony Tony Chopper's (One Piece) Monster Point possesses a tremendously hardened body to the point where superhuman attacks like Rankyaku and Shigan simply bounce off of him.
Bobby Funk (One Piece) was born with a very powerful body, where even a Busoshoku/Armament Haki imbued axe snapped when it struck him.
As a Diamond Human, Jozu (One Piece) can assume a diamond form, hardening it further by using Busoshoku/Armament Haki.
Charlotte Linlin/Big Mom (One Piece) tremendously tough physique makes her completely immune to bullets and cannonballs…
…or being electrocuted by a tremendous lightning bolt...
...with no damage at all.
Kaido's (One Piece) sheer power makes him invulnerable to almost any injuries. Even if he wants to, he cannot be harmed.
Monkey D.Luffy (One Piece) using Busoshoku/Armament Haki - Koka to harden his rubbery body to incredible levels.
Saitama's (One-Punch Man) superhuman durability allows him to withstand a punch from the Sea King to his head with only a flinch.
Escavalier (Pokémon) possesses a steel-hard carapace naturally, and uses Iron Defense to further harden its armor-like defense.
Tetsuhito Kagiroi (Tenjho Tenge) possesses an iron-hard body due to his clan's women drinking iron sand every night.
Yaya (Unbreakable Machine Doll) can harden any aspect of her body, even if they are not attached to her at the time, rendering her invulnerable to physical attacks.
Gatasubal (Yu Yu Hakusho) is composed entirely of a hybrid metal of combined steel and living cells, making it very difficult to pierce through it.