- a common English prefix meaning 2. In statements of frequency, bi- and semi- have become confused and it isn't always clear what a word like "bimonthly" means. This is how it's supposed to work: in adverbs of frequency, bi- means "every two." Thus a biweekly payroll is paid once every two weeks, a bimonthly magazine is published once every two months, and the U.S. House of Representatives is elected biennially (every two years). For something that happens twice per time unit, use semi-.
Dictionary of units of measurement. 2015.