Biennial means every two years. Biannual means twice a year — but it is so often misused for the other meaning that it has become genuinely ambiguous. If the frequency matters, write "twice a year" or "every two years" and remove the doubt entirely.
The core difference
Two words for two different frequencies, and one of them can no longer be trusted.
- biennial — every two years: a biennial conference.
- biannual — twice a year, in principle.
- semi-annual — twice a year, unambiguously.
Why to avoid biannual
Because readers genuinely do not know which you mean, and half of them will guess wrong. This is a case where the correct word is still the wrong choice: if a reader has to look it up, the sentence has failed. Write the frequency out.
The botanical sense
In gardening, a biennial is a plant that completes its life cycle over two years — growing in the first, flowering and setting seed in the second. That is a precise technical use and is worth knowing, since it explains why the two-year sense is the firmly established one.
Frequently asked questions
Does biannual mean twice a year or every two years?
Twice a year, in principle — but it is so often used for the other meaning that it is best avoided.
What should I write instead?
"Twice a year" or "every two years". Semi-annual is also unambiguous for twice a year.
What is a biennial plant?
One that completes its life cycle over two years — growing in the first and flowering in the second.