nounplural noun lustra/ˈlʌstrə/ , plural noun lustrums
1Roman History
A purificatory sacrifice made after a census every five years.‘The book is dedicated to Nygren's wife Irmgard "in thankfulness for seven lustrums of theological cooperation."’- ‘37 years ago my first secretary was a lady who has remained my friend for nearly 8 lustrums (4 decades) and today is a leading lady lawyer in Los Angeles who has never lost a case in court.’
2 rare A period of five years.
Origin
Late 16th century Latin, probably from luere ‘to wash’.
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