lustrum

noun2

1. •
   (historical) A lustration: a ceremonial purification of the people of Rome performed every five years after the census.
2. •
   (literary) Synonym of quinquennium: Any 5-year period.

   • We push Time from us, and we wish Him back; / Lavish of Lustrums, and yet fond of Life; / Life we think long, and short; Death seek, and shun; […] — 1742