Immortal

Immortals are beings who cannot die through any conventional means, which separates them from mortals. This category includes the Gods, the Titans, Monsters and some Demigods as well. There have also been instances where a mortal has been gifted with immortality by the gods, usually through either great heroism (like Hercules) or through marriage to a god (such as Psyche or Ariadne). The Hunters of Artemis are gifted with immortality so long as they keep their vow of maidenhood and do not fall in battle. Nymphs and satyrs can be considered immortal in the sense that they will never die of age. They are, however, subject to death by other means.

Characteristics
Immortals are immune to death in any conventional form. They do not age, nor can they die from poison or disease. They are immune to all weapons not forged from magical metals such as celestial bronze or stygian iron. If their bodies are somehow harmed or destroyed they will eventually reform, though the length of this process can vary from days to centuries. Almost all immortals have supernatural abilities far surpassing those of humanity, and they are typically hidden from human view by the Mist.

Limits
Immortality does, however, have its limits. If the body of an immortal being is damaged enough they may be weakened to a point where they can no longer reform, even to the point of not being able to gather a complete consciousness. They can also be sealed away in prisons such as the pit of Tartarus. They can even "fade" away into nothingness, ceasing to be should they lose the will to live, which has happened to several immortal beings such as Helios, Pan, and the brothers of Briares.