Happening twice per year or every six months.
From 'bi-' meaning two, 'annual' from Latin 'annus' meaning year, plus '-ly' to form an adverb. Common in English since the 18th century.
Confusingly, 'biannual' can mean either twice a year OR every two years depending on context—it's one of English's most notorious ambiguous words that confuses business writers everywhere.
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