Occurring, happening, or done once every two years; at two-year intervals.
From 'biennial' (Latin 'biennium' + -alis 'relating to') + -ly (adverb suffix), meaning the manner or frequency of biennial events.
The word 'biennially' sounds formal enough that many people avoid it, but environmental reports and government documents use it constantly because it's precise—no confusion between 'every two years' and 'twice a year'.
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