The term immortal has traditionally referred to those who will Live forever,those who always will be. Gaming requires a somewhat more practical definition, however: immortals are simply those who will never die of natural causes.They may be slain in combat, or killed by some natural disaster (earthquake,tomado, black hole, supernova-the usual stuff), but they never die of old age. Not all immortals are deities; many campaigns, for example, play elves as immortal, but they are certainly not deities.
A deity’s immortality is assured because deities don’t rely on their physical forms for life, but rather on their reservoirs of primal base. As long as a deity has a portion of primal base in which to house his or her consciousness, that deity will continue to exist. Deities’ material forms only place constraints on some of the things that they can do.