Is It Biannual or Biennial?

Biannual means twice in one year. Biennial means once every two years.

So a meeting that happens twice a year is a biannual meeting. A competition that takes place every two years is a biennial competition.

As the terms are often confused, I suggest you use: ‘every 6 months‘ and ‘every two years‘ —if they fit.

So instead of talking about biannual and biennial meetings, say:

There will be a review meeting every six months.
The IESC meeting is held every two years.

I find that so much easier to understand, don’t you?