Word Study
lustrum
CIDE DICTIONARY
lustrum, n. [L. Cf. 2d & 3d Luster.].
A lustration or purification, especially the purification of the whole Roman people, which was made by the censors once in five years. [1913 Webster]
OXFORD DICTIONARY
lustrum, n. (pl. lustra or lustrums) a period of five years.
Etymology
L, an orig. purificatory sacrifice after a quinquennial census
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