Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
biannual, biennial, semiannual
The bi- prefix on biannual “divides into two,” so that biannual usually means “twice a year.” Biennial comes from a Latin word whose prefix bi- meant “two,” so biennial means “every two years,” as in a biennium, “a two-year period.” Semiannual also means “every half year,” or “twice a year.” Biannual is almost a hopeless case, conveying only the idea of two, but not specifying when; semiannual is a little more helpful, suggesting as it does “half-yearly.” Make certain that context makes your meaning clear, no matter which word you use; in fact, given the potential for confusion, half-yearly and two-yearly may be better than any of these. See also BI-.
