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Tip 14: Biannual/Semiannual & Biweekly/Semiweekly

When to use ‘biannual/semiannual’ and ‘biweekly/semiweekly’

‘Biannual’ and ‘semiannual’ both mean twice a year.
‘Biennial’ means every two years.
Biweekly meetings occur every two weeks.
‘Semiweekly’ meetings occur twice a week. Since most people can’t keep them straight, just say “every other week” or “twice weekly.”

A note from our Editor

Question: “As ‘bimonthly’ means twice a month, is there a short term for ‘every two months’?”
Answer: According to the Handbook of Technical Writing (ninth edition), bi- / semi- When used with periods of time, bi- means “two” or “every two,” as in bimonthly, which means “once in two months.”

When used with periods of time, semi- means “half of” or “occurring twice within a period of time.” Semimonthly means “twice a month.” Both bi- and semi- normally are joined with the following element without a space or hyphen.

So-bimonthly = every two months, and semimonthly means twice a month.

I think the best way to keep this straight is NOT to use the term, but rather to state the occurrence specifically.

I also went to the ‘Ask Oxford’ website at and found the following:
Does bimonthly mean ‘twice a month’ or ‘every two months’?

I’m afraid it means both! But in the publishing industry, it is used fairly consistently to mean ‘every two months.’ The same ambiguity affects biweekly and biyearly. If you want to be absolutely clear, use a phrase such as ‘twice a week’ or ‘every two years.’