biannual
adj
1. Occurring or produced, etc twice a year.
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biennial
adj
1. Said of an event: occurring once in every two years.
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However in another dictionary, they sort of make the two words “biannual and biennial” interchangeable.
bi·an·nu·al ( P ) Pronunciation Key (b-ny-l)
adj.
1. Happening twice each year; semiannual.
2. Occurring every two years; biennial.