A lūstrum (IPA: [ˈluːs̠t̪rʊ̃ˑ], plural lūstra) was a term for a five-year period in Ancient Rome.
It is distinct from the homograph lustrum (/ˈlʊstrəm/ LUSS-trəm): a haunt of wild beasts (and figuratively, a den of vice), plural lustra (/ˈlʊstrɑː/ LUUS-trah).
[1] The lustration was originally a sacrifice for expiation and purification offered by one of the censors in the name of the Roman people at the close of the taking of the census.
Lustrum (from luo, Ancient Greek: λούω) is a lustration or purification of the whole Roman people performed by one of the censors in the Campus Martius, after the taking of the census was over.
It is highly probable that the recurrence of such a cycle or great year was, from the earliest times, solemnized with sacrifices and purifications, and that King Servius did not introduce them, but merely connected them with his census, and thus set the example for subsequent ages, which however, as we have seen, was not observed with regularity.