A saeculum is a length of time roughly equal to the potential lifetime of a person or, equivalently, 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.
Source: Saeculum – Wikipedia