What are “Biannual,” “Biennial,” and “Semiannual”? Rules of Usage

There’s actually no difference between biannual vs. semiannual. They both describe twice-yearly events. Biennial, on the other hand, means every other year.
“Biannual” is an adjective used to describe events occurring twice a year. In order to fully understand this term, we need to know where it derives from. The prefix “bi-,” meaning twice, comes from Latin, while “annual” refers to one year.

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