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– 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, every other Friday.
Congressional elections are held biennially, on even-numbered years.
The U.S. celebrated the bicentennial of its 1776 founding in 1976.
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).