The prefixes bi- and semi- are different. Bi- means two and semi- means half. You can remember that by remembering that a bicycle has two wheels and semisweet chocolate is only half sweet. Bifocals have two lenses, and semiannual sales happen twice a year.
Knowing the Definitions Doesn't Necessarily Help
Still, at first glance, knowing the meaning of those prefixes doesn't necessarily help. Is a biweekly meeting two times a week or every two weeks?
Grace and Bert both wrote in to ask about the word biweekly. They both think it means every two weeks, and they're right. The definition of biweekly is every two weeks, and the definition of semiweekly is twice a week, or said another way, every half week, if that helps you remember. Bimonthly is every two months, and semimonthly is every half month. The Grammar Girl podcast is now coming out semiweekly.
Few People Know the Difference
But Grace and Bert both noticed that people don't seem to know the difference and are using biweekly to mean both twice a week AND every two weeks. “When did this change of definition happen?” Grace asks.
Avoid Biweekly and Semiweekly
I don't know exactly when the change happened, but I know that it did happen. Every style guide I checked recommended avoiding words such as biweekly and bimonthly and instead just saying twice a week or every other week (1, 2, 3, 4). It's more clear.
You can feel smart if you know the difference between biweekly and semiweekly, but if you write your invitations using those words half the people will probably show up on the wrong day, and that's no way to run a meeting--unless you're running a grammar society or a word-lovers club; then your members would probably appreciate the challenge.
