A semiannual event occurs twice a year, while a biennial event happens every two years.
A biannual event can have two meanings, either twice a year or every two years.
What's the Difference
In What’s the Difference, the distinctions between similar words are explored. Words such as cemeteries and graveyards, cougars and pumas, oceans and seas are explained, as well as seltzer water and club soda, milk and buttermilk, buffalo and bison.
Words that we frequently use interchangeably often have subtle differences, and sometimes have significantly different meanings. This book provides a fun look into the array of words in the English language that have similar meanings
What's the Difference
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“Crossing a body of water is an adventure, whether riding a small ferry to Isle Royale National Park, or a larger ferry to one of the Greek islands. “