biannual, biennial, semi-annual

Semi-annual events occur twice a year.

• Come to our semi-annual sale: it happens only twice a year!

Biannual events occur either every two years or twice a year.

• The biannual ICAN conference is held every two years and brings together Canadian pastel artists from across the country.
• Our biannual journal appears in June and December.

To avoid confusion, use semi-annual rather than biannual or twice a year instead of biannually.

• Our journal is a semi-annual (not biannual) publication.
• Our journal appears twice a year (not biannually).

Biennial events occur every two years. Biennial can also refer to a two-year period (such as for a plan or budget).

• We prepared the university’s biennial plan for the next two years.

In gardening, biennial plants have a two-year life cycle.

• Use biennials for the new border; we won’t have to plant again next year.