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.
Bimonthly = every two months
Semimonthly = 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 checked the Oxford Lexico website to answer the question:
Does bimonthly mean ‘twice a month’ or ‘every two months’?
I’m afraid it means both! 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.’
