tricennium
A period of thirty years.
rare
As early as the second decade after his entry into Italy Theodoric made all illegal or irregular acquisitions that had taken place prior to this fixed date [28 August 489] subject to the thirty-year statue of limitation (tricennium).
Where it comes from
From Latin trīcennium, from trīcennis (“30-year”) + -ium, from trīciēs (“30 times”) + annus (“year”) + -is (“forming compound adjectives”). Equivalent to tricennial + -ium.
Adapted from the Wiktionary entry tricennium (CC BY-SA 4.0).