Immortality

The ability to live forever.

There are several degrees of immortality, including limited, where a being does not age, but may still die of injury or disease, situational, where a being is immortal while in a specific sitation, such as while in statis, or while in possession of a magical item which grants immortality, and true immortality, where an individual will live forever except in very exceptional circumstances.

Beings and races with extremely long lifespans, such as Asgardians, are sometimes described as immortal by shorter-lived races, but this is innacurate.

See Also
Healing Factor
Resurrection
Users
Agamotto
Ancient One (previously)
Carina Tivan
Clea
Hive
In addition, all members of the following species or groups possess immortality
Celestials
Dark Elves
Light Elves
Elders of the Universe
Faltine
Many-Angled Ones
The Lords of the Splinter Realms 
Orisha