Lifelink

Lifelink is a static ability that lets you gain life when one of your creatures (or other permanents) deals damage.

Comprehensive Rules
702.15. Lifelink

702.15a Lifelink is a static ability.

702.15b Damage dealt by a source with lifelink causes that source’s controller, or its owner if it has no controller, to gain that much life (in addition to any other results that damage causes). See rule 119.3.

702.15c If a permanent leaves the battlefield before an effect causes it to deal damage, its last known information is used to determine whether it had lifelink.

702.15d The lifelink rules function no matter what zone an object with lifelink deals damage from.

702.15e If multiple sources with lifelink deal damage at the same time, they cause separate life gain events (see rule 118.9).
 * Example: A player controls Ajani’s Pridemate, which reads “Whenever you gain life, you may put a +1/+1 counter on Ajani’s Pridemate,” and two creatures with lifelink. The creatures with lifelink deal combat damage simultaneously. Ajani’s Pridemate’s ability triggers twice.

702.15f Multiple instances of lifelink on the same object are redundant.