Thesaurus / lustrums
Roget's 21st Century Thesaurus, Third Edition Copyright © 2013 by the Philip Lief Group.

EXAMPLE SENTENCES FROM THE WEB

We who have lived fifteen lustrums have already witnessed the dissolution of our world.
He had been gone three days, and, in their lapse, Theo felt as if three lustrums had passed.
THEOMRS. FRANCES HODGSON BURNETT
They counted their ages and eras, which they inscribed in their books every twenty years, in lustrums of four years.
VESTIGES OF THE MAYASAUGUSTUS LE PLONGEON
Dost thou not envy that smirk young knave with his five lustrums, though it goes hard with him to purchase his kid-gloves?
THE BERTRAMSANTHONY TROLLOPE