What’s the Difference between Biannual & Biennial?

• “Biannual” refers to something that happens twice in one year, while “biennial” means something occurs once every other year.
• “Semiannual” is interchangeable with “biannual” and also refers to things that happen twice a year.
• Add context to your statement to help clarify whether something happens biannually or biennially, or just say “twice a year” and “every other year” instead.

“Biannual” & “Biennial” Meanings

“Biannual” refers to something that happens twice within one year. “Biannual” can be used as an adverb (“I visit my parents biannually”) or as an adjective (“the biannual clearance sale”). “She cleans out her closet biannually.” “I have my biannual doctor appointment next week.”

“Biennial” describes something that occurs once every 2 years. Like “biannual,” it can be used as an adjective (“our biennial trip to Europe”) or as an adverb (“I renew my membership biennially”). Unlike “biannual,” “biennial” can also be a noun (“It’s time to plant my biennials”) or even a proper noun in the case of recurring events like the Venice Biennale (a famous art exhibition that happens every 2 years). “I only have enough money to go on vacation biennially.” “It’s time for our biennial family reunion!” In gardening terms, a biennial is a plant with a life cycle of 2 years, usually blooming or flowering during its second growing season.

“Semiannual” Meaning

“Semiannual” is a synonym for “biannual.” It also refers to something that happens twice a year and is interchangeable with “biannual.” It can be used as an adjective (“a semiannual sale”) or as an adverb (“she visits him semiannually”). “Come to our committee’s semiannual meetings to stay in the loop on our progress.” “This job is great! We get bonuses semiannually!”

Communicating the Correct Meaning

Add context to clarify what you mean by “biannual” or “biennial.” For example, if you’re concerned someone may misunderstand what you mean if you say “We meet biennially,” add some detail to make it clear you mean “every other year” and not “twice a year.” You could say something like, “We only meet biennially since our projects take over a year to complete.” “We try to get together biannually, usually around New Year’s Day and Father’s Day.” “The forget-me-nots I planted are biennials, so they won’t bloom until next year.” Context is extra important for words like “biweekly” or “bimonthly,” which can mean both “once a week/month” and “once every other week/month.”

Replace “bi-” with “semi-” to say something happens twice in a period. Since “biannual” and “biennial” are so commonly mixed up, help your reader or listener understand what you mean and try using “semiannual” to describe things that occur twice in one year. Similarly, use “semiweekly” or “semimonthly” to clarify when something happens twice in one week or one month, rather than once every other week or month.

Use other, clearer phrases like “twice a year” or “every other year.” If remembering the difference between “biannual” and “biennial” is daunting to you or your readers and listeners, ditch both words altogether. Say “twice a year” instead of “biannual,” and “every other year” or “every 2 years” in place of “biennial.” These phrases are universally understood and help avoid miscommunication. “I need to check in with you twice a year.” “He updates his entire wardrobe every other year.”

Etymology of “Biannual” & “Biennial”

“Bi-” is a Latin prefix meaning “two,” “double,” or “once every two.” “Bi-” by itself does not indicate whether something happens twice within a given period or only once every 2 periods, which explains the confusion between words like “biannual” and “biennial.” This is also why words like “biweekly” or “bimonthly” need clarification—they can mean both “twice a week/month” and “once every other week/month,” depending on the context. For example, a bimonthly meeting could mean a meeting that happens twice a month or a meeting that only happens once every 2 months. The meaning is totally dependent on the user’s intent. The Latin prefix “semi-” is less vague. It means “half,” so something that happens semiannually occurs every half-year (although today, “semi-” can also mean “partially” or “somewhat”).

Both “-annual” and “-ennial” come from the Latin word annus ("year"). The vowel change from “a” to “e” doesn’t have any impact on the meaning—it just comes from an old Latin pronunciation rule. In fact, the difference in meaning between “biannual” and “biennial” is completely arbitrary since there’s nothing in the roots of either word to set them apart!