Occurring twice a year or every two years (usage varies, which can cause confusion).
From Latin 'bi-' (two) + 'yearly' (from Old English 'gear', meaning year). The ambiguity in English comes from inconsistency in how 'bi-' prefixes work.
Biyearly is confusing because 'bi-' can mean both 'twice' and 'every two'—'biweekly' suffers the same problem, which is why experts now recommend saying 'twice yearly' or 'every two years' instead.
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