The term Immortal can be properly applied to any undying creature - including Familiars and Dragons. Many people use it in this way but it once meant only the ancient and undying spirits who left the void at the beginning of time. There are many class of Immortal, each different in nature, but related by descent or origin.
Some, such as the Divines, are Immortals who have importance to mortals or reality, proclaiming themselves gods of a sort. Other immortals are nothing more than a restless chaos, energy ever changing, with a ever shifting personal identity.