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]

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