Immortality is the ability to live forever without fear of aging or dying. Characters who possess this ability usually don't develop it fully until adulthood, as many of them maintain adult physiques, and appear to be between the ages of 25-35. Generally, immortal characters do not require sustenance such as food or drink. Some don't even require extended periods of rest, and are almost always active. Although a character may be immortal, this does not mean that they cannot die. It only means that they will never expire due to natural causes. While some characters such as Gods and Cosmic Beings are true immortals (meaning that they will never die), others are restricted by various limitations or require alternate methods to maintain their immortality. Vandal Savage for example, is one of the oldest living beings on the planet Earth. However, to maintain his immortality, he must harvest compatible organs from his own descendents, or else he risks complete biological shutdown. Another example is the character of Ra's al Ghul. Ra's maintains his current age and vitality so long as he can immerse himself in the mystical fluids found in a Lazarus Pit. It has been shown that Ra's will continue to exist well into the 31st century.
Gods and other cosmic entities find their immortality tied to humanity or some other abstract construct. So long as one of these qualifiers continues to exist in the cosmos, the being in question will continue to live. For example, Dream of the Endless is effectively immortal so long as there is at least one living being in the universe capable of dreaming.
Many Godlike beings from various Earth-based pantheons are immortal so long as humanity continues to value and/or believe in them. This particular limitation appears to vary from pantheon to pantheon and is not necessarily a staple for all Gods. For instance, in the Kid Eternity ongoing comic book series, the Gods of Olympus awakened after centuries of slumber, because humanity began to believe in them once again. These Gods should be distinguished from the more commonly seen Greek deities from the Wonder Woman mythos, whose lives appear to continue in perpetuity regardless of external circumstances. The Gods of Apokolips and New Genesis are also true immortals, however, they can still be killed, although the means by which to do so usually requires an extreme effort.
Some characters achieve immortality through mystical means. Circe and Mordru for example, while not Godlike beings, are powerful sorcerers who have used their abilities to extend their natural lifespan. The Amazons of Themyscira are immortal so long as they remain on native soil. Once they leave Themyscira, they will begin to age as a normal human and will eventually die of natural causes. Upon returning to Themyscira however, their immortality is immediately restored.
The Guardians of the Universe are immortals. They seem to reach a certain age and then cease physically aging entirely, though as they were once an entire species, they are capable of being killed, though with their immense powers of Willpower, it is difficult to do so. Their race has existed since just after the beginning of the Universe.
There are strong indicators that Superman may also be immortal. Theoretically, so long as he has access to a constant source of yellow solar radiation, he will not age or die. In the continuity of the Smallville television series, it has been established that Clark Kent may live (seemingly) forever.
Immortality should not be confused with Resurrection, which represents a character's ability to return from the dead. While such characters are effectively immortal, they can usually be killed through conventional means.