The word "longevity" is sometimes used as a synonym for "life expectancy" in demography - especially when it concerns someone or something lasting longer than expected (an ancient tree, for example) - however, the two words have slightly different definitions. In a similar dichotomy to "precision" and "accuracy", "longevity" refers to the average number of years that a single person lives, whereas "life expectancy" refers to the number of years that the average population lives. This is illustrated by the fact that a drastic increase in life expectancy (due to a larger portion of the population beginning to live longer) may be accompanied by a small increase in the overall longevity of the population.