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bi-annual v biennial

Don't say that: 

Bi-annual when you mean every other year
Biennial when you mean twice a year.

Do say this: 

Bi-annual when you mean twice a year
Biennial when you mean every other year.

Why?: 

Let's illustrate this with regard to plants.

A bi-annual flowers twice each year.

A biennial flowers once this year, not next year then once the year after and so on.

The prefix "bi" means two or, in this case, twice.

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