FromLatintrīcennium, fromtrīcennis(“30-year”) +-ium, fromtrīciēs(“[30]]times”) +annus(“year”) +-is(“formingcompoundadjectives”). Equivalent totricennial +‎-ium.

tricennium (pluraltricenniaortricenniums)

1. (rare) Aperiod ofthirtyyears.
   • 1979, Thomas J. Dunlap, transl. Herwig Wolfram asHistory of the Goths,p. 298:

     As early as the second decade after his entry into ItalyTheodoric 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).

   • 2021, Gavin Lucas,Making Time, page48:

     With a site I have been working with inIceland, I am dealing with 30-year time units ortricennia, and even if it is noPompeii, there are still features that we know represented much shorter time scales—the construction of a fireplace that could have been accomplished in a day, the placing of a coin under a timber sill beam that only took a few seconds.

• annum (1 year),biennium (2),triennium (3),quadrennium (4),quinquennium (5),sexennium (6),septennium (7),octennium (8),novennium (9),decennium (10),vicennium (20),centennium (100),quincentennium (500),millennium (1000),decamillennium (10,000),centimillennium (100,000),millionennium (1,000,000)

• (Classical Latin)IPA^(key):[triːˈkɛn.ni.ũː]
• (modern Italianate Ecclesiastical)IPA^(key):[t̪riˈt͡ʃɛn.ni.um]

Fromtrīcennis(“30-year”) +‎-ium(“-ium:formingabstract nouns”), fromtrīciēs(“30times”) +annus(“year”) +-is(“formingcompoundadjectives”).

trīcennium n (genitivetrīcenniīortrīcennī);second declension

1. tricennium, a30-yearperiod

Second-declension noun (neuter).

¹Found in older Latin (until the Augustan Age).

• English:tricennium

trīcennium

1. genitivemasculine/feminine/neuterplural oftrīcennis

• “tricennium”, inCharlton T. Lewis and Charles Short (1879)A Latin Dictionary, Oxford: Clarendon Press
• tricennium inGaffiot, Félix (1934)Dictionnaire illustré latin-français, Hachette.