Just as you don’t want to confuse complement with compliment or less with fewer, you also want to be very clear when it comes to describing the frequency of events. Do you ever confuse biannual with biennial? It’s a common error, considering the root of these words and how similar they sound. Let’s have a look at the difference.
Let’s start with semiannual. If you break this word down into its two main parts, it’s fairly easy to discern its meaning. Semi- denotes the “half” of something. When you talk about a semicircle, you are talking about half a circle (180-degrees). Annual denotes that this word has something to do with a year. This is the same root that gives us the word anniversary. In this way, semiannual (or semi-annual, if you prefer) indicates that an event is taking place every half a year. In other words, a semi-annual sale would take place every six months (twice a year).
Where there can be a great deal of confusion is with the word biannual. Breaking this word into its two main parts can actually make the matter even more confusing. The annual part of the word remains the same as in semiannual, but the bi- portion suggests that the number two is involved. For instance, a bicycle has two wheels. The supposedly bipartisan politics of Barack Obama address both major political parties. Given these meanings, you may be inclined to believe that a biannual event takes place every two years. And you’d be wrong. A biannual event actually takes place twice a year, just like a semiannual event.
How, then, do you denote an event that takes place every two years? That’s where the term biennial comes into play. If you get your eyes tested by your optometrist every two years, you go for biennial checkups.
Probably one of the best ways to avoid further confusion in the future is to minimize your use of the word biannual, as that’s the word that seemingly (and incorrectly) straddles between the meanings of “every six months” and “every two years.” Stick to using semiannual and biennial, and your meaning should be clearer.