What to KnowUsing biannual without enough context often leads to confusion since “every two years” or “twice a year” are both possible meanings of this word. Biennial can be used, with its meaning of “every two years,” but its similarity to biannual can cause confusion. Semiannual is a good alternative and is used consistently to mean “twice a year.”
There is nearly always confusion about the meaning of the word biannual: does it mean “twice per year” or “every two years”? This confusion comes from the fact that the prefix bi- can mean both “two” or “twice.” For words like bicycle or bisect it’s fairly clear that that meaning is “two,” as is “two wheels” or “cut in two.”
But when bi- is connected with a word that describes a period of time, such as weekly, monthly, or annual, there is rarely enough context to let us know whether, say, “twice a week” or “every two weeks” is the intended meaning. The word biennial is usually used to mean “occurring every two years,” but its similarity to biannual makes a misunderstanding possible.
The use of semi- to mean “half” is consistent and clear, and semiannual has become a useful alternative that means “occurring twice a year.” It’s always best to make your intended meaning clear if using one of these words that begins with bi-, and remember that a short formulation like “every two weeks” or “twice a week” can sometimes be a better choice.
