Lustrum

Webster's Dictionary

(n.) A lustration or purification, especially the purification of the whole Roman people, which was made by the censors once in five years. Hence: A period of five years.

The Nuttall Encyclopedia

A sacrifice for expiation and purification offered by one of the censors of Rome in name of the Roman people at the close of the taking of the census, and which took place after a period of five years, so that the name came to denote a period of that length.