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.

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

Semiannual' vs 'Biannual': What's the Difference?

What Is Bi-annual? When to Hyphenate - Elite Editing

Semiannual: Definition, Example, Vs. Biennial and Biannual

Biannual or Biennial? You May Be Doing it Wrong!

Why are you like this? : r/NonPoliticalTwitter

Makes perfect sense, have one word that means two things : r/CasualUK

What Is Bi-annual? When to Hyphenate - Elite Editing

Biannual: Unveiling the Differences: Semiannual vs Biannual - FasterCapital

Anniversary Latin-Derived Term Other Terms Comments: Biennial Every Second Year, PDF

Calaméo - Writing Style Guide And Preferred Usage For Do D Issuances 2013

Biannual: Unveiling the Differences: Semiannual vs Biannual - FasterCapital

A Few Commonly Misused Words

How did the word biannual come to mean both once every two years and twice a year? Isn't it a useless word now? - Quora