An immortal is a creature in the Tortall universe that lives indefinitely unless killed. Gods live forever; they cannot be killed. Lesser immortals, those that live among mortals, can be killed. The Badger explained this to Daine and Numair. The Human Era began after all the immortals were trapped by mages in the Divine Realms. Centuries later, in 447 HE, Carthaki mages found the spells used and released many immortals back into the human realms, primarily in Tortall. The Eastern and Southern Lands were plunged into The Immortals War.
A decision of the Great Gods in 452 HE ruled that the immortals already in the Mortal Realms could remain, but the rest would be confined to the Divine Realms. This coincided with the end of the Immortals War.
Different species of Immortals vary widely in temperament, intelligence, and abilities.