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The Collaborative International Dictionary of English v.0.48:

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. [1913 Webster]