Bi- vs. semi-

What does bi- mean?

Bi- is a prefix which means two.

For example:

• Let’s set up a biweekly meeting (every two weeks).

What does semi- mean?

Semi- is a prefix which means half.

For example:

• Let’s set up a semiweekly meeting (twice a week).

What’s the difference between bi- and semi-?

Bi- means two while semi- means half, meaning a biweekly meeting would be held every two weeks while a semiweekly meeting would be held twice a week. However, biannual and semiannual both mean twice a year. To say once every two years, use biennial instead.