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.”
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 http://oxforddictionaries.com/page/meaningbimonthly?view=uk and found the following:
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’.
Again, it is better to just be specific and forgo the shortened term if you want to be certain that you communicate the correct term!