a common English prefix meaning 1/2. In statements of frequency, bi- and semi- have become confused and it isn't always clear what a word like "semimonthly" means. This is how it's supposed to work: in adverbs of frequency, semi- means "twice every" or "every half." Bells on a ship ring semihourly (every half hour) and the tides usually occur semidiurnally (twice a day); a semiweekly newspaper is published twice in a week; a semimonthly payroll is paid twice every month; and days and nights have the same length semiannually (twice a year). For something that happens once every two time units, use bi-.