Biannual is an adjective used to describe something that happens twice a year, or something that happens every two years.

// The art show is a biannual event that won’t happen again for two more years.

// The group holds biannual meetings in December and July.

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“About 200 miles of trails make up The Alabama Coastal Birding Trail along Alabama’s Gulf coast. These comprehensive trails span both Baldwin and Mobile counties, following the coastline, wetlands, and backwaters—all crucial stopover habitats for migratory birds. Visit in the fall or spring to see part of the beautiful biannual journey for yourself.” — Catherine Jessee, Southern Living, 8 Mar. 2025

When we describe something as biannual, we can mean either that it occurs twice a year or that it occurs once every two years. So how does someone know which particular meaning we have in mind? Well, unless we provide them with a contextual clue, they don’t. Some people prefer to use semiannual to refer to something that occurs twice a year, reserving biannual for things that occur once every two years. This practice is hardly universal among English speakers, however, and biannual remains a potentially ambiguous word. Fortunately, English also provides us with biennial, a word that specifically refers to something that occurs every two years or that lasts or continues for two years.