['lʌstrә]
lustrum的复数

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.