n. pl. lus·trums or lus·tra (-trə)
1. A ceremonial purification of the entire ancient Roman population after the census every five years.
2. A period of five years.
[Latin lūstrum; see LUSTER.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.