A saeculum is a period of time lasting about the length of a long human life - approximately 80 years. Within written history, the earliest people to measure time in saecula were the Etruscans - around the ninth century BC, a sibyl prophesied that their society would last for 10 lifetimes. The Etruscans began to measure time in this way: among the first to be born at the founding of a city or civilization, the one who lives the longest marks, upon his death, the end of the first saeculum; among those born on that day, the one who lives the longest marks the end of the second, and so on.
Approximately 10 long lives after the prophesy, the Romans invaded Etruria.
Rome, as it was wont to do, adopted the saecular philosophy as their own: a new prophesy expected Rome to last 12 saecula (which, amazingly, it did - counting from the founding in 509 BC to the fall of the Visigoths in AD 410), and the saecular games were arranged so that every Roman citizen would be alive to see them at least once. Though the Romans employed the concept of a civil saeculum (100 solar years), their history is marked with references to natural saecula (about 80-100 years, or one lifetime).
The modern American seculum is made up of four "seasons", or turnings, each lasting twenty years or so: a high, an awakening, an unraveling, and a crisis, followed by another high.
This used to be longer; see Turnings in American history.