When you break apart the word biannual, it’s easy to remember what it means. The prefix bi-, meaning “two,” tips you off that this is something that happens twice. And you’re probably already familiar with annual, which describes something that happens once a year. Put it together and you have biannual: twice a year. Whether you’re describing formal events or the frequency of your haircuts, you can use biannual anytime you need to say that something happens twice a year.