Immortality means living forever, never dying. In stories and myths throughout history, people have imagined what it would be like to have immortality: to watch centuries pass while you remain unchanged, to see civilizations rise and fall, to never grow old or sick.

The ancient Greek gods were immortal, which set them apart from humans, who had to accept their limited time on Earth. Vampires in legends possess immortality but pay a price for it. The Greek myth of Tithonus tells of a man granted immortality but not eternal youth, so he aged forever without dying, a cautionary tale about getting exactly what you wish for.

Scientists sometimes use the word differently. Cancer cells achieve a kind of immortality by continuing to divide and grow when normal cells would stop. Some jellyfish species can reverse their aging process and theoretically live indefinitely, making them biologically immortal, though they can still be eaten by predators or die from disease.

People also speak of immortality in a different sense: the idea that great achievements, books, or acts of courage can make someone's memory immortal. When we say Shakespeare achieved immortality through his plays, we mean his work lives on even though he died more than 400 years ago. This kind of immortality is about being remembered forever, not about never dying.