Immortality is a physical trait, typically magical in origin, which temporarily or permanently stops the natural ageing process. Though immortal beings are still vulnerable to disease, poison, or injury, their body will remain the same physical age as it was when it was made immortal for an indefinite period of time. Some creatures and races (such as Elders and Archons) are naturally immortal, but most races (such as Humani) can only become immortal through artificial or magical means. Immortality such as this can be bestowed by a handful of powerful beings, though it can be easily revoked, causing the body of the immortal to reach its true physical age in a matter of seconds, resulting in death. Some spells are capable of granting eternal life - a spell contained within the codex has kept Gilgamesh alive for well over 10,000. The Codex also contains a recipe for the Philosopher's Stone, which grants immortality for a single month before the recipe changes and a new brew must be procured.
Elders often grant immortality as a gift to humans who have helped them. Since this type of Immortality can be revoked with a mere touch, immortals often become servants to Elders. Dark Elders use this leverage to control agents such as Dr. John Dee and Niccolo Machiavelli.
Some powerful beings are capable of bestowing immortality onto humans. Some Elders, such as Aten, Tammuz, and Quetzalcoatl are capable of granting immortality. The Earthlords Isis and Osiris also wield this power, which they used to control Dr. John Dee for centuries.
Elixirs of ImmortalityThe Codex also contains a recipe for a temporary elixir of immortality which lasts for roughly one month. The recipe changes every time, and using an old recipe will actually speed up the ageing process. This method of immortality has sutained Nicholas and Perenelle Flamel for centuries, though once Dr. John Dee steals the codex from them in The Alchemyst the two begin to age at a rate of roughly one year per day (or faster, when their auras are used).
The Comte de Saint-Germain claims to have discovered and perfected a recipe for immortality that did not need to be renewed every month. However, given Saint-Germain's dubious character, this may not be entirely true.
Some spells can grant immortality. Abraham made Gilgamesh immortal (albeit in a flawed way) using a spell contained within The Codex. It is likely that Abraham made Tsagaglalal immortal using a spell as well, however she retains the use of her aura and her memory, which would indicate that the spell was further perfected after Gilgamesh.
Some substances can grant immortality. Injecting vampire blood grants immortality (and does NOT turn the human into a vampire) - Joan of Arc gained immortality upon receiving a blood transfusion from Scathach which was meant to save her life after she was nearly burned at the stake.