Some people believe that the word “bimonthly” refers to an event that takes place every two months. Others claim it actually describes something that happens twice per month. Well, at the risk of delighting and upsetting both sides of the argument, I’m here to tell you the truth is that “bimonthly” can be used as an adjective, noun, or adverb to imply either twice a month or every two months.
The prefix “bi-” means “two,” “twice,” “double,” etc., and comes from the Latin bi- with a similar definition. It indicates two of something but is vague with regard to frequency. This creates a bit of a headache when affixing it to time-based words such as “monthly” or “weekly.” A “bi-” prefix can imply “two times each” or “every two,” both being technically correct in an etymological sense.
The best way to alleviate this confusion, short of coining a new word, is to use a different prefix that already exists: “semi-.” The “semi-” prefix means “half,” suggesting the topic is divided into two equal parts. “Semimonthly” means an event happens twice per month. If it’s used in the context of an announcement for a lineup of bimonthly and semimonthly events, for example, folks will know which occur every two months and which they should show up for twice a month.
However, for yearlong periods, things are a bit different. English has two distinct terms: “biannual” (“twice per year”) and “biennial” (“every two years”). The similar spellings might trip people up on the difference between the terms, but it’s likely some people may not even realize “biennial” exists as a distinct word. This is another case where you can swap in the “semi-” prefix to help differentiate between the two — “semiannual” is more commonly used in advertisements and announcements.
But to answer our original question, “bimonthly” can be used for either time period, and now you have an etymology lesson in your pocket to counter any naysayers.
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