Etymology
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biannual (adj.)

also bi-annual; "occurring every six months, twice a year," 1837; see bi- + annual (adj.). Distinguished in sense from biennial, but the distinction is etymologically arbitrary. Related: Biannually; bi-annually.

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Definitions of biannual

biannual (adj.)
occurring or payable twice each year;
Synonyms: semiannual / biyearly / half-yearly
From wordnet.princeton.edu