The English prefixes bi- and semi- are often mixed up by native speakers. A semi-annual reading of this lesson will help more than a bi-annual one.
 

Bi-

Bi- comes from the Latin meaning two. When used with a temporal word, bi- means "every two" or "every other."

This magazine is published bi-weekly, on the 1st and 15th of the month.

Congressional elections are held biennially, on even-numbered years.

The U.S. celebrated the bicentennial of its 1776 founding in 1976.
 

Semi-

Semi- comes from the Latin meaning half. When used with a temporal word, semi- means "twice."

Our semi-annual meetings are in January and July.

I get paid semi-monthly, on the 5th and 19th of each month.

The semi-weekly deliveries occur on Monday and Thursday.

 
The Bottom Line

The confusion between bi- and semi- occurs because both prefixes are related to the concept of two. But bi- means something that happens every other (week, month...), or every two (weeks, months...), while semi- indicates something that happens twice every (week, month) or every half (week, month).
 

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