Bi- is a prefix meaning two or twice; semi- is a prefix meaning half. There is debate surrounding the actual definition of bi- being both two and twice, although semi- is always half. Assuming bi- can be two or twice, bimonthly can be every two months or twice a month, which is a bit confusing and can lead to a big difference in meaning. For example, think about being paid bimonthly: Is it twice a month or every other month? Semimonthly, however, is always twice a month.  To avoid confusion, it is better to use specific wording, such as “The magazine is printed every other week,” or “The magazine is printed every other month.” The same pointer applies to biweekly vs. semiweekly and biannually vs. semiannually.