A "lustrum" is a period of how many years?

A "lustrum" was a term for a five-year period in Ancient Rome. The term is still in use today.

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. The sacrifice was often in the form of an animal sacrifice, known as a "suovetaurilia".

These censuses were taken at five-year intervals, thus a lustrum came to refer to the five-year inter-census period. "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. As this purification took place only once in five years, the word "lustrum" was also used to designate the time between two "lustra".

The first "lustrum" was performed in 566 BC by King Servius, after he had completed his census, and afterwards it is said to have taken place regularly every five years after the census was over. In the earliest period of the republic, the business of the census and the solemnities of the "lustrum" were performed by the consuls.