Biennial is a term referring to a period of two years, much in the same way centennial refers to 100 years. If an event occurs every two years, it can be said to occur biennially. This is commonly used in reference to legislative agendas in U.S. states (often as biennium), as many representatives are elected to two-year terms.
The word is used to clarify meanings where the terms semiannual or biannual might otherwise be used. Both of those terms have become ambiguous, sometimes meaning something that occurs twice a year or otherwise meaning something that occurs once every two years. In this case, prefixing bi- (meaning "two") to annual can either indicate that occurs twice annually, or that it has a period that is twice that of an annual event. The inverse is true with semi- (meaning "half").
Today, major dictionaries define both semiannual and biannual as something that occurs twice a year. However, in contrast, bimonthly is given a double meaning: either twice a month, or once every two months.
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