Look up saeculum in Wiktionary, the free dictionary.
A saeculum is a length of time roughly equal to the potential lifetime of a person or the equivalent of the complete renewal of a human population. The term was first used by the Etruscans. Originally it meant the period of 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.
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.
Strauss, William and Howe, Neil, The Fourth Turning, Broadway, 1997. Details the saecula in the past 600 years of Anglo-American history, from the Protestant Reformation to a forecast of the Millennial generation entering adulthood in the 2010s.