n. 人口普查, 五年时间

Lustrum \Lus"trum\, n.; pl. E. {Lustrums}, L. {Lustra}. [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. Hence: A period of five years.