Thanos (Marvel) the Mad Titan cursed by Death to live forever.
"I intend to live forever, or die trying."
"Some pirates achieved immortality by great deeds of cruelty or derring-do. Some achieved immortality by amassing great wealth. But the captain had long ago decided that he would, on the whole, prefer to achieve immortality by not dying."
― Terry Pratchett, The Color of Magic
The power to never age and recover from almost any injury. Opposite to Mortality.
Users possess an infinite life span, and can shrug off virtually any kind of physical damage. Some users are the defensive type, simply preventing such damage from appearing (invulnerability/protection), while others are the regenerative type, surviving and quickly recovering from anything you throw at them.
Some may only possess the power of:
Chronolock: Exist outside normal space-time continuum, and thus immune to what happens in it.
Death: No body to destroy. The soul, one that is not of a physical being, does not inherit physical characteristics, such as concepts of destruction, therefore cannot be destroyed; in short, the user cannot die a second time. Most common form of immortality.
Teitoku Kakine (A Certain Magical Index) achieved a form of immortality by creating a human tissues (and a new body) out of his Dark Matter.
Ladylee (A Certain Magical Index) is an immortal, in that when she grew weary of living, she sought to use powerful magic to kill her, which did not work.
Tenzen Yakushiji (Basilisk) having his symbiote "eat" away his wounds and restoring any ravages of time or battle, even reattaching his head by sealing the cut.
Creed Diskenth (Black Cat) possesses the God's Breath nano-machines within his body, regenerating even fatal wounds in seconds and maintaining his youth, thus granting him immortality aside from any brain damage being irreparable.
Sōsuke Aizen (Bleach) gained immortality after fusing with the Hōgyoku.
C.C (Code Geass) is immortal.
V.V (Code Geass) is immortal.
Garlic Jr. (Dragon Ball) made a wish via the Dragon Balls for eternal life, and will regenerate from any and all injuries seamlessly.
Kagerō (Flame of Recca) using a forbidden spell that opens a time portal, but it traps her outside of space-time, rendering her completely immortal.
The Truth (Fullmetal Alchemist) is invincible, immortal and invulnerable.
Utsuro (Gintama) possesses immortality by harnessing the Altana energy of Earth to prevent aging and recover from wounds and diseases.
Yūta (Mermaid Saga) is a 500 years old immortal since unwittingly eating mermaid's flesh.
Mana (Mermaid Saga) is a 15 years old immortal since being fed mermaid's flesh.
Masato (Mermaid Saga) is an 800 years old immortal since eating mermaid's flesh.
Orochimaru (Naruto) considers himself immortal with his Living Corpse Reincarnation to transfer his soul to another body and his Cursed Seals as anchors of his conscious.
Hidan's (Naruto) main advantage is his inability to die by physical damage, though he is vulnerable to death by lack of nutrient.
Kakuzu (Naruto) attained a form of immortality (though he denies to think of it as such) by tearing hearts out of others and integrating them into himself, extending his lifespan. He kept five inside him at all times.
Madara Uchiha (Naruto) claims he has achieved complete immortality due to hosting the Shinju, as he regenerated form his torso being blown apart. Only when the tailed beasts were all pulled out of him did he die.
Kaguya Ōtsutsuki (Naruto) is immortal, in that she has tremendous regenerative powers, and that the only way to defeat her is to seal her person away by splitting her chakra into the nine tailed beasts.
Gemma Himuro (Ninja Scroll) putting his severed body parts back together, even his head is possible, rendering him immortal.
Due to her race, Jibril (No Game No Life) has age 6407 years old, she has incredibly vast knowledge and high magical abilities, in two words; she live with gathering many old and new knowledge, in other words; she is no longer age nor die.
Rin Asogi (RIN ~Daughters of Mnemosyne~) is immortal, due to a magic spore from Yggdrasil. she can even handle more alcohol than a normal person.
Free (Soul Eater) is a werewolf from the Immortal Clan, and therefore, immortal. He can only be harmed and killed by the "Witch-Hunt".
Kojō Akatsuki (Strike the Blood) is revealed to be immortal, even by vampire standards after regenerating from complete decapitation.
Tōta Konoe (UQ Holder) cannot regrow limbs unless they are completely destroyed, but otherwise is immortal and can reattach any of it, including his head.
Karin Yūki (UQ Holder) has one of the highest ranked forms of immortality, stating that she's "not permitted to get hurt or die"
Elder Toguro (Yu Yu Hakusho) stated that his regenerative powers enables him from dying. This prevented him from dying from Kurama's torturous Sinning Tree.
Zeref (Fairy Tail) was cursed by Ankhseram with his contradiction curse which gives him uncontrollable Death Magic and Immortality.
Porky Minch (Earthbound) has abused Time Travel so much that his body is stuck in the current timeline and cannot age nor die.
It is believed that Ganondorf (The Legend of Zelda) is immortal due to the Triforce of Power.
Clockwerk (Sly Cooper) has kept himself alive for millennia thanks to his cybernetic body and his jealousy and hatred of the Cooper Clan.
Solaris (Sonic the Hedgehog) is a super-dimensional life form and the Sun God of Solenna, and exists in all timelines that he is immortal unless he is killed simultaneously in every temporal point.
Presea Combatir (Tales of Symphonia) is immortal and invulnerable because of a combination of her exsphere and her special ability Suppress
Kaguya Houraisan (Touhou Project) drank the Hourai Elixir, which grants her immortal in every sense of the word: she does not age, is immune to disease, and can regenerate from even being completely disintegrated.
Though he can be imprisoned and sealed away, Grima (Fire Emblem) can only be truly and permanently killed by his own hand.
Snow White (Valkyrie Crusade) is a immortal princess that is always trying to die,but anything works.
Pariah Dark (Danny Phantom) is the powerful immortal, former king of ghosts.
Peter Griffin (Family Guy) Peter Griffin has survived many life threatening situations and came back unscathed.
Ernie the Giant Chicken (Family Guy) always comes back for a rematch despite Peter Griffin always dealing a fatal blow on Ernie.
Lord Voldemort (Harry Potter) acquired immortality by splitting his soul and hiding the fragments in various objects as anchors, though when his body was destroyed, he existed as a spectral form that many others would prefer death over.
Fawkes (Harry Potter) is a phoenix, who will be reborn with all of his memories intact upon death, and thus immortal, being the only known creatures in the wizarding world to have natural immortality.
Adam Monroe (Heroes) possessed immortality due to his tremendously advanced regeneration ability, though once the ability is taken away from him, he died within seconds.
The Dog Talisman (Jackie Chan Adventures) grants its master invincibility.
Nathan Young (MisFits) is immortal.
General Immortus (Teen Titans) knows the strategy of every battle in history because he was there see it.
Starscream (Transformers G1) possesses an immortal Spark, soul energy, meaning even if his physical vessel is destroyed he will live on.
Jason Voorhees (Friday the 13th) can only be truly and permanently killed by his own family members.
The Beast (Doctor Who) claims to have existed before our universe was created
Candyman (Candyman) has lived for centuries
Ashildr AKA Me (Doctor Who) is effectively immortal due to being given a Mire computer chip.