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 lus·trum /ˈlʌstrəm/
 人口普查,五年時間

 Lus·trum n.; pl. E. Lustrums L. Lustra   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.
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 lustrum
      n 1: a period of five years
      2: a ceremonial purification of the Roman population every five
         years following the census