In English, lustrum is a rather rare literary word that means a period of five years, a quinquennium.
What is very remarkable, a comparison of different editions will show, that the fundamental doctrines of a whole “Series of Grammars, English, Latin, and Greek,” may so change in a single lustrum, as to rest upon authorities altogether different.
The Grammar of English Grammars, by Goold Brown, 1851.
My guess is that more people have met lustrum as the title of the Robert Harris bestseller about politics in ancient Rome than have encountered it in real life. Robert Harris gave his book that title because it covers the five years following 63 BC. Americans won’t know the book by that title, as the book’s US publisher decided that potential readers would be put off by such a strange Latinate word and renamed it Conspirata, even though that sounds even more like Latin and which isn’t an English word at all.
There’s a link between a five-year period and a census because in classical Rome the population was counted every five years, as it still is in some countries today, such as Australia and New Zealand. The census was carried out by two magistrates called censors, the city’s tax assessors, as part of a valuation of the property of Romans for tax purposes (an early sense of census in English was to a type of poll tax). When the enumeration was over, one of the censors held a ceremony called a lustratio or lustrum in the Campus Martius at which a pig, a sheep and an ox were sacrificed in the presence of the people. The sacrifice was called a suovetaurilia by the way, a Latin mouthful formed from sus, pig, ovis, sheep, and taurus, ox. By extension lustrum came to mean both the ceremony and the period between two censuses.
Dictionaries don’t agree about the source of lustrum. Some argue it comes from luere, to wash, because the ceremony originally involved ritual cleansing; others say it’s from lustrare, to purify or brighten, which would make lustrum a close relative of lustre and some other English words. Others warily include variations on “ultimate origin unknown”.
Censor, by the way, has its modern English meaning because the magistrates who conducted the census and assessed people for taxes were also responsible for maintaining public morals. Busy men.