Lustrum

from WordNet (r) 3.0 (2006)

lustrum
    n 1: a period of five years
    2: a ceremonial purification of the Roman population every five
       years following the census

from 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]