There was often much confusion about this in the past because as you said it can mean multiple things. We seem to have gone away from any proper etymological use of the word 'bi' and have defined (for the most part) biweekly to be every two weeks, bimonthly to be twice a month, biannually to be twice a year (that one maybe not). Legal documents that I see don't use those terms to avoid confusion.
Frustratingly, "biannual" can also mean twice a year or every two years. Fortunately there is the "biennial" which unambiguously means every two years.
Are you sure about that? I'm from Canada and distinctly remember the travel ads urging us to head on down to participate in the bicentennial celebrations, meant to celebrate the second century of that country's founding.
One of the 'trivia' things on the display in our elevator was about how Websters had a listing for 5 years that wasn't actually a word, 'dord'. Like come on, now you can't even trust words in the dictionary?!