FromLatinbiennium, frombiennis(“2-year”) +-ium(suffix forming abstract nouns), frombi-(“two”) +annus(“year”), q.v.

• (General American)IPA^(key):/baɪˈɛn.i.əm/
• (Received Pronunciation)IPA^(key):/bʌɪˈɛn.ɪ.əm/

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• Hyphenation:bi‧enn‧i‧um

biennium (pluralbienniumsorbiennia)

1. Aperiod oftwoyears,particularly forpurposesinvolvingintercalation orfiscalcalculations.

   Coordinate terms:annum,triennium,quadrennium,quinquennium,sexennium,septennium,octennium,novennium,decennium,vicennium,tricennium,centennium,quincentennium,millennium,decamillennium,centimillennium,millionennium

   • 1704, Giles Strauchius [i.e.,Aegidius Strauch II], “Of the Lunar Cycle”, inRichard Sault, transl.,Breviarium Chronologicum. Or A Treatise Describing the Terms and Most Celebrated Characters, Periods and Epocha’s Used in Chronology. By Giles Strachius, D.D. and Publick Professor in the University of Wittebergh. Now Done into English from the Third Edition, in Latin. By Richard Sault, F.R.S., 2nd corr. and enl. edition, London: Printed for A. Bosvile at the Dial and Bible againstSt. Dunstan's Church inFleet-street,→OCLC, book II (Of Chronological Characters), § 1,pages93–94:

     TheGreeks being taught by their Oracles, that their accuſtomed Sacrifices were to be offeredκατά τρία, which they underſtood as if their Year were to be regulated by the Sun, and their Days and Months were to be adjuſted by the motion of the Moon; were always ſolicitous, how by certain Periods they might reduce the diſagreeing motions of the Luminaries to aThird ſomething in which they might agree: Hence in the ancient times they are ſaid to have uſed aBiennium, intercalating every other year: But fault was found with this, and 'twas ſucceeded by aQuadriennium; upon the return of which the Olympic Games were celebrated.

   • 1858,Herodotus,George Rawlinson andHenry Rawlinson, translators and editors, “The First Book, Entitled Clio”, inThe History of Herodotus. A New English Version, Edited with Copious Notes and Appendices, Illustrating the History and Geography of Herodotus, from the Most Recent Sources of Information; and Embodying the Chief Results, Historical and Ethnographical, which have been Obtained in the Progress of Cuneiform and Hieroglyphical Discovery.[...] In Four Volumes, volume I, London:John Murray,Albemarle Street,→OCLC, footnote 2,page178:

     No commentator onHerodotus has succeeded in explaining the curious mistake whereby the solar year is made to average 375 days. That Herodotus knew the true solar year was not 375, but more nearly 365 days, is clear from book ii. ch. iv. It is also clear that he must be right as to the fact that the Greeks were in the habit of intercalating a month every other year. This point is confirmed by a passage inCensorinus (De Die Natal. xviii. p. 91), where it is explained that the Greek years were alternately of 12 and 13 months, and that thebiennium was called "annus magnus," orτριετηρίς.

   • 2007, Digambar Bhouraskar, “Growth and Structure of EPTA[Expanded Programme of Technical Assistance]”, inUnited Nations Development Aid: A Study in History and Politics, New Delhi: Academic Foundation,→ISBN,page138:

     During thebienniums 1963–64 and 1965–66, expenditure on regional and inter-regional projects also registered a substantial growth. In fact, the share of regional projects in the total programme during thebienniums 1963–64 and 1965–66 was higher than the 12 per cent deemed desirable by the TAC [Technical Assistance Committee].

   • 2014, Joel W. Paddock, “Local and State Political Parties”, in Donald P. Haider-Markel, editor,The Oxford Handbook of State and Local Government, Oxford:Oxford University Press,→ISBN,page231:

     Although ideology is related to the propensity for a state to adopt stringent changes to campaign finance law, culture does not have a substantive effect. However, he [Christopher Witko] finds support that a scandal in the previousbiennium is associated with changes to campaign finance regulation in the currentbiennium, in concert with the findings of [Beth] Rosenson (2005) on the adoption of legislative ethics reforms.

   • 2016, Manabu Saeki, “Ideology of Partisan Voters and Congressional Members”, inThe Phantom of a Polarized America: Myths and Truths of an Ideological Divide, Albany, N.Y.:State University of New York Press,→ISBN,page58:

     Ideology of Republican voters demonstrates a significant, if not enormous, rightward shift during this period. In contrast to Republican legislators, the conservative shifts by Republican voters betided in a few, rather than all,biennia, interrupted by otherbiennia with no ideological shift.

• biennial
• biennially

• “biennium,n.”, inOED Online ⁠, Oxford:Oxford University Press,2022.

• (Classical Latin)IPA^(key):/biˈen.ni.um/,[biˈɛnːiʊ̃ˑ]
• (modern Italianate Ecclesiastical)IPA^(key):/biˈen.ni.um/,[biˈɛnːium]

Frombiennis(“2-year”) +‎-ium(suffix forming abstract nouns), frombi-(“two”) +annus(“year”), q.v.

biennium n (genitivebienniīorbiennī);second declension

1. biennium

Second-declension noun (neuter).

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

• → Catalan:bienni
• → English:biennium
• → Italian:biennio
• → Romanian:bieniu
• → Spanish:bienio

Regularly declined forms ofbiennis.

biennium

1. genitivemasculine/feminine/neuterplural ofbiennis

• “biennium”, inCharlton T. Lewis and Charles Short (1879)A Latin Dictionary, Oxford: Clarendon Press
• “biennium”, inCharlton T. Lewis (1891)An Elementary Latin Dictionary, New York: Harper & Brothers
• "biennium", in Charles du Fresne du Cange’sGlossarium Mediæ et Infimæ Latinitatis (augmented edition with additions by D. P. Carpenterius, Adelungius and others, edited by Léopold Favre, 1883–1887)
• biennium inGaffiot, Félix (1934)Dictionnaire illustré latin-français, Hachette.