If something is “bimonthly”, does it occur twice a month or once every two months? In fact, just to demonstrate how slippery the English language can sometimes be, the correct answer would be “either”. The Oxford English Dictionary definition is very straightforward :
appearing or taking place twice a month or every two months.”
It’s fair to say that “every two months” would be the more common usage of the word, but there’s no guarantee that a reader will assume one meaning or the other. If you want to use the term it may be worth spelling out (at least once) what you mean by it. Or just use different words, such as “every two months”. In the US, the word “semimonthly” is often used to mean “twice a month” but this is not a common word in other parts of the world, such as the UK.
The same problem occurs with words for other periods of time. Bi-weekly, for example, can mean both “twice a week” and “once every two weeks”. So, strictly speaking it isn’t clear whether a “bi-weekly” publication will appear once or four times over a two week period.
Similarly a biannual magazine could be published once every six months or once every twenty-four. On the other hand, the word “biennial” is unambiguous and means “once every two years” or “lasting two years”. This word is thus often used to refer to plants that only flower or fruit in their second year of life.
The ambiguity of language can be a part of its beauty, especially if you are writing fiction or poetry. But if you want to be sure of conveying a precise meaning, there are some words that are best avoided.