For those that didn't explore the more information link... probably depending on where you are, they use:

• Semi-annual for 2x a year
• Biennial - for every two years.

2 hours ago, Myr said:

For those that didn't explore the more information link... probably depending on where you are, they use:

• Semi-annual for 2x a year
• Biennial - for every two years.

 

Where I work, we write semi-annual reviews of progress (every six months) and complete bi-annual re-assessments every two years.  

Dictionary.com says both. See link.  *shrugs*  As I said to @Carlos Hazday, it's probably also a regional thing as well as both @Valkyrieand I are near each other in the "Inland Northern" dialect region.  https://www.washingtonpost.com/blogs/govbeat/wp/2013/12/02/what-dialect-to-do-you-speak-a-map-of-american-english/

1 hour ago, Backwoods Boy said:

I saw this in use a couple of days ago.  I wish I could remember where because it posed the same unsolvable ambiguity as shown in the information link.  Unless you're a lawyer or a politician, why use a word that has two opposite meanings? 

For the same reason, to say something which can mean whatever is convenient at a particular time.