Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
bi-, semi- (prefixes)
When prefixed to nouns meaning periods of time, bi- means “every two,” so that bimonthly means “every two months,” biweekly “every two weeks.” Semi-, when prefixed to words meaning periods of time, means “half,” or “halfway through,” so that semimonthly means “every half month,” or “twice a month,” and semiweekly means “every half week,” or “twice a week.” Unfortunately, when you bisect a line you divide it into two equal parts, and a good many other words begin with a bi- that means “into two,” rather than “every two.” So a rare but naggingly recurrent meaning of bimonthly is also, alas, “twice a month.” Best advice: when you use bi- and semi- words, let context help define them, even at the cost of some redundancy. See BIANNUAL.
