Lustrum

Lustrum (from luere, 'to purify' or 'expiate'), the solemn offering made for expiation and purification by one of the censors in name of the Roman people at the conclusion of the census. The animals offered in sacrifice were a boar (sus), sheep (ovis), and bull (taurus), whence the offering was called Suovetaurilia. As the census was quinquennial, the word lustrum came to mean a period of five years.