The prefix bi- means “two” or “twice.” It attaches to the front of words to show that something has two parts, happens twice, or involves two of something.

A bicycle has two wheels (while a tricycle has three and a unicycle has one). Someone bilingual speaks two languages fluently. When you bisect something in math class, you cut it into two equal parts. A biweekly newspaper comes out twice a week, or every two weeks, depending on the context.

You'll find this prefix in science words like bilateral (having two sides), in everyday words like binoculars (using two eyes at once), and in words describing time periods like biannual (twice a year) or bicentennial (a 200th anniversary).

Watch for potential confusion: biweekly and bimonthly can mean either “twice a week/month” or “every two weeks/month.” If you're not sure which meaning someone intends, context usually helps.