Definify.com
Webster 1913 Edition
Lustrum
Lus′trum
,Noun.
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.
Webster 1828 Edition
Lustrum
LUS'TRUM
,Noun.