Longevity is defined as "long life" or "a great duration of life." The term comes from the Latin word longaevitās. In this word, you can see how the words longus (long) and aevum (age) combine into a concept that means an individual who lives a long time. The most important part of this definition is the comparative nature of it. Long life implies longer than something—and that something is the average lifespan.