It means both, twice a week and every two weeks. It's confusing but what part of english isnt?

I'm wondering the same thing about Bible. Does it mean twice per Ble or every other Ble?

I was taught that the "bi" prefix was a multiplier and "semi" was a divider.

That meant biweekly, bimonthly, biannually were every 2 weeks, months, years and semi-weekly, semi-monthly, semi-annually were every half a week, half a month, and half a year.

Then the real world intruded and I've been confused ever since. About the only time I hear "semi" and "bi" used on a regular basis the way I expect is with pay periods. Biweekly is every two weeks and semi-monthly is twice a month.

Canada, by the way.

PS: I suppose bisexual and semi trailers also fit my expectations.

It means both. Welcome to English.

It's because of British English, and the fact that American English seems to have dropped a word which is caused confusion.

Bi-weekly means two times a week.

Fortnightly means every 2 weeks. But American English seems to have lost the word fortnightly, so there is this ambiguity now.

this is TIL, for me. "fortnightly" almost always solves it.

I always think the rule was "bi-" for "two" like bicycles VS semicycles.

dictionary people say it is up to the sayer to avoid confusion.

It can be either, actually. Yes, it's stupid.

The real answer is to solve this by using different terms. For instance, "twice per week" or "every other week".

Don't try to get anyone to agree on a definition, it's just begging for problems.

An old word that fell out of use to describe a two-week period is "fortnight."

It should make a come back, but I fear the current generations would always misspell it for... reasons.

I prefer to use "semiweekly" for twice in a week, and so on for other periods.

Wait, so bi-weekly and bi-monthly mean almost the same thing (every 14/15 days)? That's insanity!

The banks use “biweekly” and “semiweekly” to avoid this exact kind of ambiguity. Biweekly would be twice a week, while semiweekly would be every other week.

It comes up in banking a lot because of payroll. If you get paid every other week, you get paid semiweekly. But if you get paid on the 1st and 15th of every month, you get paid bimonthly.

Fortnightly means every two weeks. Bi weekly means twice a week.

There was often much confusion about this in the past because as you said it can mean multiple things. We seem to have gone away from any proper etymological use of the word 'bi' and have defined (for the most part) biweekly to be every two weeks, bimonthly to be twice a month, biannually to be twice a year (that one maybe not). Legal documents that I see don't use those terms to avoid confusion.

I think the conflict is between invisibly different sub-word groupings. I think of them as "(biweek)ly" = "happens every biweek" = "happens every two weeks, vs. "Bi(weekly)" = "happens twice as much as weekly" = "happens two times every week".

That doesn't really help the ambiguity, so I prefer other ways of describing the recurrent timing of events when there isn't anything obviously disambiguating them - for example, if I create a digital calendar event and name it "biweekly event", the existence/nonexistence of repeated calendar events makes it obvious what is meant.

Compare semi-weekly.

This is semantically correct and everyone else is wrong. Semi weekly is every other week. The widespread confusion on this is the surest sign that degeneracy prevails in the world.

The word for "biweekly" you are looking for is "semi-weekly".