The Question:

How often is a bimonthly meeting held?

The Answer:

Bimonthly is a problematic word, meaning either "occurring once every two months" or "occurring twice a month." Which one is meant depends on context. According to Merriam-Webster's Dictionary of English Usage, in the world of publishing, the former definition is almost invariably the intended one; a bimonthly magazine comes out six times a year. In the world of education, the latter definition is often intended, with bimonthly meetings taking place roughly every two weeks. The same problem applies to biweekly.

If you do mean "twice a month" or "twice a week," you might be better off either writing those phrases, or using the unambiguous "semimonthly" and "semiweekly." If you mean "every two months" or "every two weeks," you might be better off using those phrases; otherwise, be sure that the context makes it clear which meaning you have in mind.

—The Fact Monster