Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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