There are many different types of immortality and technically all beings are in part "immortal" due to Life, Death and Rebirth but are considered "mortal" in the sense their life-force will move to a new world / universe and body upon "death", with radically different appearances and natures (yet still with repressed memories of "past lives").
True Immortals can not physically die nor grow old or be harmed by toxin and disease, however if they die of some other cause (magic or cosmic-level force (such as Deicide) ) they will undergo the usual "rebirth" of a "mortal".
In contrast beings that have "Revolving Immortality" will be reborn in the same universe and body, over and over until the destruction of their native universe - each time they are reborn they retain perfect memory of self as well as the same physical form (some rare versions will adapt upon being "reborn", making them resilient to death by the same means).
In general all immortals will "die" when their native universe is destroyed, being "reborn" as new beings in the next - the exceptions to this rule are beings that are in Chrono-Lock (all "God Force" and "Shadow Force" are by default Chrono-Locked in addition to their omnipotent status as Supreme Beings).
Items Which Grant This Power
- Becoming a Balance-Keeper or Agent of Imbalance grants one immortality and Eternal Youth, as Balance-Keepers are part of the Multiverse itself
- Toons are Immortal due to the fact they are "living memories" - they grow more powerful in relation to how many people believe in them but can never truly die as long as their is psychic energy for them to cling onto.