Definition of "lustration"

It will be remembered, that this beautiful Island was sacred to the ancient Greeks, from having been the birth-place of Apollo and Diana. None were born or died there—the mothers and the dying were carried to the neighbouring islet of Rhane. Solemn expeditions, with much priestly pomp, were frequently made from Athens to enforce this ordinance, particularly to propitiate the Gods in time of public calamity. Our era refers to the celebrated lustration, at the time of the Peloponnesian war, during the plague of Athens.

1830, Felicia Hemans, A Song of Delos, footnote, from The Winter's Wreath, 1831, page 49