Biannual means happening twice a year. A school might hold biannual book fairs, one in fall and one in spring. A dentist recommends biannual checkups, spacing them about six months apart. Some companies hold biannual sales in January and July.

When something is biannual, it occurs two times within a single year.

This word often gets confused with biennial, which means happening once every two years. A biennial event happens half as often as a biannual one. Think of it this way: biannual means twice per year, while biennial means once per two years. If your town has a biannual parade, you'll see it twice this year. If it has a biennial parade, you'll see it this year, skip next year, then see it again the year after that.

Some people also use semiannual to mean the same thing as biannual, though semiannual makes the “twice a year” meaning a bit clearer.