What does biennially mean?
Biennially is a formal adverb meaning once every two years. It describes events that recur on a two-year cycle — conferences held biennially, reports published biennially, plants flowering biennially. Derived from Latin biennis ('lasting two years'), from bis ('twice') plus annus ('year'), it belongs to the same family as annual, perennial, and biennial. Its chief hazard is ambiguity: because 'bi-' can be read either as 'two' or as 'twice,' many people confuse biennially with biannually (twice a year), a mix-up so widespread that usage guides frequently advise replacing the adverb with the plain phrase 'every two years.' In practice, biennially appears mostly in institutional and academic contexts — governing bodies, exhibitions such as the Venice Biennale, and scheduled reviews — where precision about frequency carries real weight. Used carefully, it lends prose an efficient, official tone; used loosely, it invites misreading, which is precisely why it rewards a clear understanding.
adverbOnce every two years; at two-year intervals. Also, less commonly, occurring twice a year (though this sense is now generally considered nonstandard or ambiguous).
- Occurring once every two years; at intervals of two years.
- (Less commonly, now often considered nonstandard) Occurring twice a year.
"The festival is held biennially, drawing artists from around the world every other summer."
"The Venice Biennale is held biennially, transforming the city into a showcase of international art."
"The committee meets biennially to review amendments to the constitution."
Because 'biennial' can mean both 'every two years' and (in older usage) 'twice a year,' it's one of English's most famously ambiguous words — even dictionaries disagree.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of biennially
Biennially derives from the adjective biennial, formed from Latin biennis, meaning 'lasting two years,' itself a compound of bis ('twice') and annus ('year'). The adjective entered English in the early seventeenth century, and the adverb followed as a regular -ly formation. It shares its root with annual, perennial, and the noun biennium ('a period of two years'). The same bis + annus construction also produced biannual, whose competing interpretation of 'twice yearly' lies behind much of the word's modern confusion.
Related word forms
How biennially is actually used
Formal register, most often found in administrative, academic, and organizational writing about meetings, elections, reports, festivals, and plant life cycles. Because of persistent confusion with 'biannually,' careful writers often substitute the explicit phrase 'every two years' when clarity matters. The secondary sense of 'twice a year' is widely discouraged by modern usage authorities.
Easily confused with biennially
'Biannually' means twice a year, while 'biennially' means once every two years — a confusion so common that style guides recommend spelling out the interval.
'Semiannually' unambiguously means twice a year, whereas 'biennially' refers to something happening every two years.