A plural form referring to multiple periods of two years or events occurring every two years.
From Latin biennium (bi- 'two' + annus 'year'), this modern form extends the classical term to create a regularized plural for contemporary use.
This word shows how old Latin terms get adapted into English plurals—sometimes we use the original Latin plural (biennia) and sometimes we anglicize it (bieennia), reflecting the messy reality of how languages borrow and adapt words.
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