Q: What's the difference between bi-annual and biennial?—Anonymous
A: I see these words treated as if they were interchangeable—most often by marketing departments—but they aren't. And marketing departments should be extra cautious, as misusing these two words could cost them quite a bit of money.
"Bi-annual" means twice a year, or two times within a 365-day period. So if you hire a heating company to do a bi-annual cleaning of your furnace, they will send someone out once in the summer and once in the winter—of the same year.
"Biennial," on the other hand, means once every two years. Therefore, if you contract the heating company on a biennial basis, they will come out to your house every other year to clean your furnace. For example, I have my gutters cleaned on a biennial basis. The gentleman who does it came out in May 2009, which means I won't see his smiling face again until May 2011.
Be sure to get these words straight. If you say "biannual" when you mean "biennial," you'll be doubling the number of times you'll have to do something in a given year (and quadrupling over a 2-year period). A mistake like that could really come back to bite you.