Immortals are any being that can live theoretically forever but aren't considered a god for whatever reason. The reasons are as varied as immortals themselves. 

Generally, immortals need a soul anchor, although dragons are the exception to this rule. A soul anchor is always made of voidgold - a mysterious metallic substance with strong ties to Kasw and the energy of the universe. Voidgold is practically indestructible, and can take many appearances. For those of angelic descent, it usually looks like halos, crowns, or other jewelry. For demons, it is a tattoo that can sometimes change shape. There are exceptions to this rule, most commonly for anchorless immortals who gain their anchors later, or for sorcerers.                                                                          For blood titans, their anchor is the planet itself.