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A saeculum is a length of time roughly equal to the potential lifetime of a person or, equivalently, the complete renewal of a human population. The term was first used by the Etruscans. Originally it meant the time from the moment that something happened (for example the founding of a city) until the point in time that all people who had lived at the first moment had died. At that point a new saeculum would start. According to legend, the gods had allotted a certain number of saecula to every people or civilization; the Etruscans themselves, for example, had been given ten saecula.
By the 2nd century BC, Roman historians were using the saeculum to periodise their chronicles and track wars. At the time of the reign of emperor Augustus, the Romans decided that a saeculum was 110 years. In 17 BC, Caesar Augustus organised Ludi saeculares ("saecular games") for the first time to celebrate the "fifth saeculum of Rome". Later emperors like Claudius and Septimius Severus have celebrated the passing of saecula with games at irregular intervals. In 248, Philip the Arab combined Ludi saeculares with the 1000th anniversary of the founding of Rome. The new millennium that Rome entered was called the saeculum novum, a term that got a metaphysical connotation in Christianity, referring to the worldly age (hence "secular").
A saeculum is not normally used for a fixed amount of time; in common usage it stands for about 90 years. It can be divided into four "seasons" of approximately 22 years each; these seasons represent youth, rising adulthood, midlife, and old age.