Webster's English Dictionary

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lustrum

lus.trum \'l*s-tr*m\ \-tr*\ n or lustrums or lus.tra [L; akin to L lustrare 
   to brighten, purify] pl  1a: a purification of the whole Roman people made 
   in ancient times after th e census every five years 1b: the Roman census  
   2: a period of five years : QUINQUENNIUM