“Biannual” and “Semiannual”

There’s one simple secret to keeping these straight: just remember that you’re specifying a period, not a frequency. “Annual” means there is one year between each occurrence, so “Biannual” means two years between each occurrence, and “Semiannual” means half a year between each occurrence (or twice each year). Same goes for “biweekly,” “bimonthly,” etc. The confusion comes because the root words (e.g. “annual,” “monthly”) specify a value of one unit each, so the frequency and period, which are reciprocal, are numerically equal under the given unit. “Once a year” and “every one year” mean the same thing, but “twice a year” and “every two years” do not. All you need to remember is that it’s the latter (“every X units-of-time”) that is generally the correct interpretation.