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.
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.
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.
Jebediah of Canaan (more commonly know as the wizard Shazam) is an immortal.