Lifelink

Lifelink is a keyword ability introduced in Future Sight. Damage dealt by a source with lifelink causes that source's controller to gain that much life.

History
Before it was keyworded in Future Sight, R&D used to call this ability spirit link, after the Spirit Link. Note, however, that that card doesn't actually have the exact lifelink ability.

Future Sight hinted at a lot of things from Magic's potential future on its futureshifted cards. For lifelink, that future was the very next set released, Tenth Edition. This ability can be traced back to the card El-Hajjâj in Arabian Nights.

Before the rules changes in Magic 2010, lifelink was a triggered ability, and it used the stack along with combat damage. This meant that its life gain if caused by combat, would arrive too late to prevent the player from losing the game due to combat damage from the same combat. It is now a static ability that causes life gain at the same time as the damage that causes it. The ability is primary in white and secondary in black. White and black have very different flavor rationales for the abilities, but both use it almost every set.

Ikoria: Lair of Behemoths introduced lifelink counters.

Knightlifelink
The silver-bordered Unsanctioned set features the variation Knightlifelink (Damage dealt by Knights you control also causes you to gain that much life.)

Rulings

 * The ability causes life gain whenever a source with lifelink deals any damage, not just combat damage.
 * All older cards with the ability "Whenever this permanent deals damage, you gain that much life." that received errata to change that ability to "lifelink" have reverted to their original wording with the exception of Loxodon Warhammer, which has been printed with the word "lifelink" in Tenth Edition and all subsequent reprintings.
 * Note that the card Spirit Link does not grant the creature it enchants lifelink. The lifelink ability has the creature's controller gain life. Spirit Link has Spirit Link's controller gain life. The same is true for Vampiric Link.

Enchantments that grant just Lifelink
One creature
 * Felidar Umbra
 * Lifelink

Creatures that grant Lifelink to Instant and Sorcery spells

 * Firesong and Sunspeaker</c> (Red spells you control)
 * Radiant Scrollwielder</c> (spells you control)
 * Soulfire Grand Master</c> (spells you control)

Trivia

 * The vigilance, lifelink, and fear keywords are the only keywords with a card of the exact name (Vigilance</c>, Lifelink</c>, and Fear</c>, respectively). Flying comes close, with the card called Flight</c>.
 * Five Instant and Sorcery spells have functional lifelink through the wording "you gain life equal to the damage dealt this way": Syphon Soul</c>, Corrupt</c>, Brightflame</c>, Rite of Consumption</c> and Enter the God-Eternals</c>. Granting them lifelink allows one to double the amount of life gained.