Biannual, semiannual - How often?

People often confuse the meanings of the terms biweekly and semiweekly. If you found out you were required to be at a semiweekly meeting, went to the first one, then showed up two weeks later, you’d be pretty embarrassed – you missed two intervening meetings! The -weeklies can be kept straight pretty easily. Just look at the prefixes: bi- means two, and semi- means half, so biweekly is every two weeks, and semiweekly is every half week, or twice per week.

Now get ready for the really confusing part.

By the above logic, you might be inclined to show up for a biannual meeting once now, in January, 2009, and then show up again in January, 2011. That’d be even worse than missing your semiweekly meeting – biannual means the same thing as semiannual, and they both mean twice a year!

If something only occurs every two years, like a House of Representatives reelection campaign, the word to use is biennial.

The worst part is that these words can be carelessly used to mean either twice per week or every other week, so we never really can tell how often that conference happens, short of calling the organizers.

Grammar Girl and Grammarphobia both recommend avoiding the combination of bi- or semi- with -weekly or -monthly. I wouldn’t hesitate to extend this prescription to the -annuals. If you mean twice a year, for clarity’s sake, just say it!

Source: The Associated Press Stylebook, 42nd ed., pp. 29-30.