Biannual vs. biennial?

Biannual means ‘twice per year.’ Biennial means ‘once every two years,’ ‘every other year,’ or ‘lasting for two years.’

Your writing, at its best

Compose bold, clear, mistake-free, writing with Grammarly's AI-powered writing assistant

What is the difference between biannual and biennial?

What do the words biannual and biennial have in common? Time and prefixes. These two confusing adjectives use the same prefix bi-, which means ‘two’ or ‘having two.’ Therefore, we use “biannual” to mean ‘occuring twice a year’ and “biennial” for ‘occuring once every two years,’ ‘every other year,’ or ‘lasting a period of two years.’  

You might recognize the prefix bi- in similar words such as bimonthly and biweekly, and these terms can be equally confusing for similar reasons. For instance, “bimonthly” means ‘occuring twice per month or every two months,’ while “biweekly” means ‘occurring every other week or once every two weeks.’ 

What does biennial mean?

According to the New Oxford American Dictionary, the adjective biennial describes something as ‘occurring once every two years,’ ‘every other year,’ or ‘occuring for a two-year period’ (“Biennial” 164”). The adverb biennially simply describes something as occurring in a biennial manner. 

Sentence examples: 

  • “Professional athletes travel the world for biennial championships.” 
  • “The museum’s biennial event features local artists from the Pacific Northwest.” 
  • “The city releases data reports biennially.” 

When describing a plant, the adjective biennial means ‘lasting for two years.’ However, the noun biennial references “a plant that takes two years to grow from seed to fruition and dies” or “an event celebrated or taking place every two years” (164). 

Sentence examples: 

  • “The pandemic forced the desert biennial to skip a year in 2021.” 
  • “There are several native biennials in Southern Arizona.” 

What does biannual mean?

The word biannual is an adjective that means ‘occuring twice a year.’ As an adverb, the term appears as “biannually.” 

Sentence examples: 

  • “Remote employers often require their employees to meet for biannual meetings.” 
  • “Summer solstice occurs once a year, but solstice, by itself, is a biannual event.”
  • “Manhattanhenge is a unique solstice event in New York that occurs biannually.” 

Etymology of biennial

The origin of biennial dates back to the 17th century, where it derived from Latin biennis: bi- ‘twice’ + annus ‘year’ +al (“Biennial” 164). 

Can we use biannual and biennial as synonyms?

Many people use biannual instead of biennial to mean ‘once every two years’ and ‘twice a year,’ effectively making these terms modern synonyms. But while common usage changes language over time, it’s important to know that “biannual” does not traditionally mean ‘every two years.’ 

Words ending with the suffix -ennial typically mean ‘occuring every X years’ or ‘lasting for X years.’ For example, there’s quadrennial (4), quinquennial (5), sexennial (6), vicennial (20), centennial (100), and so forth. 

The word annual actually means ‘occuring once every year,’ so pairing bi- and -annual directly translates to ‘twice in one year,’ and not ‘once every two years.’ 

What to use instead of biannual and biennial?

As noted by Garner’s Modern English Usage, the best synonym for “biannual” is “semiannual,” because it also means ‘occuring twice a year’  (Garner 108). 

Instead of: “The Blue Dot Sale is the store’s best biannual event.”

Try this: “The Blue Dot Sale is the store’s best semiannual event.”

Since we can use “biennial” for multiple meanings, sometimes it’s best to replace the adjective with a more direct phrase. 

Instead of: “We travel to Idaho for biennial family reunions.”

Try this: “We travel to Idaho for family reunions every two years.” 

Instead of: “The school admits new students on a biennial basis.” 

Try this: “The school admits new students every other year.” 

Instead of: “He is serving a biennial term in office.”

Try this: “He is serving a two-year term in office.” 

FAQ: Related to biannual vs. biennial

What’s the difference between annual, biennial, and perennial plants?

The words “annual,” “biennial,” and “perennial” are botany terms that describe a plant’s life cycle (i.e., seed, germination, seedling, mature plant, flower, fruit, death, repeat). 

Annual plants are the most common type of seed or plant you’ll find in a  store because they only live for up to one year. Popular examples in the United States include peas, wheat, corn, watermelon, and marigold. 

Biennial plants have a life cycle of two years, where the first year is spent developing vegetative structures before flowering and producing fruit the following year. Common biennials include vegetables like onions, parsley, and carrots.

Plants that live longer than two years are called “perennials” because they are ‘continually occurring.Perennial plants are herbaceous or woody, which references their structural ability to withstand winters without dying. Popular garden perennials include basil, strawberry plants, potatoes, lavender, and roses. 

What’s the difference between triannual and triennial?

The adjective “triannual” is an obsolete spelling of “triennial,” which means ‘occurring three times a year’ or ‘a period of three years.’ 

Do words need -ennial to describe periods of time?

Several English words describe periods of time, and they don’t always end with the suffix -ennial. Notable examples include: 

  • A period of two years is called a biennium
  • A period of ten years is called a decade.
  • A period of 25 years is quadricentennial
  • A period of 50 years is called five decades or semi-centennial.
  • A period of 100 years is called a century.
  • A period of 1,000 years is called a millennium.
  • A period of a million years is called a megannium. 

Additional reading

If you enjoyed learning about biannual vs. biennial, check out The Word Counter’s lessons on topics, such as:

Test Yourself!

Test how well you understand the difference between biannual and biennial with the following multiple-choice questions. 

  1. True or false?: Writers can use semiannual as a direct synonym for biannual. 
    a. True
    b. False
  2. The adjective biennial can describe something as ______________. 
    a. Occurring once every two years
    b. Occurring for a two year period
    c. Occuring every other year
    d. All of the above 
  3. The adjective biannual traditionally describes something as ______________.
    a. Occurring twice a year
    b. Occurring every two years
    c. Occurring for a two year period
    d. A and B
  4. When describing plant cycles, the noun biennial means ______________.
    a. Producing a harvest every two years
    b. Achieving dormancy twice a year
    c. A life cycle of two years
    d. Producing a harvest twice a year
  5. The noun ___________ is a two year period. 
    a. Biennial
    b. Biannual
    c. Biennium
    d. Biennially


  1. A
  2. D
  3. A
  4. C
  5. C


  1. Annual.” Lexico, Oxford University Press, 2021.
  2. Biannual.” The Dictionary, Merriam-Webster Inc., 2021.
  3. “Biennial.” The New Oxford American Dictionary, 3rd ed., Oxford University Press, 2010, p. 164. 
  4. Biennium.” The Dictionary, Merriam-Webster Inc., 2021.
  5. Garner, B. “Biannual; biennial; semiannual.” Garner’s Modern American Usage, 3rd ed., Oxford University Press, 2009, pp. 114.
  6. Perennial.” The American Heritage Dictionary of the English Language, 5th ed., Houghton Mifflin Harcourt Publishing Company, 2021. 
  7. Triannual.” The Dictionary, Merriam-Webster Inc., 2021.