Immortality in Tolkien's work is the concept of living for an infinite length of time, unless suffering a violent death.

Only beings such as the Wizards, Ainur and Elves are immortal. Men, Dwarves and others cannot live forever and thus are subject to aging and death.

In the latter half of the Second Age, Sauron's corruption of the Numenorian King Ar-Pharazôn made him believe that if he could invade and conquer the Blessed Land he could wrest immortality from them, but he was wrong and Numenor was destroyed and he was imprisoned.