Lustrum, in ancient Rome, was originally a sacrifice for expiation and purification offered by one of the censors of Rome in the name of the Roman people at the close of the taking of the census, which took place every five years. The name came to mean a period of five years, in the same way that a decade is a period of ten years. This was also a part of the process by which new senators were appointed to the Roman Senate.
Quotation: "Thus I turned over the last ten years in my mind, and then, fixing my anxious gaze on the future, I asked myself, 'If, perchance, thou shoulds't prolong this uncertain life of thine for yet two more lustres ... coulds't thou ... face death ... hopefully...?'" -- Francis Petrarch, "Letter to Dionisio da San Sepolcro [The Ascent of Mount Ventoux]" ca. 1336. Trans. J.H Robinson/H.W. Rolfe.