It usually means every other week. It sounds like it should mean twice a week, but I can only recall one time it was used that way, and of course it resulted in a lot of confusion.
Bingo. As this thread shows, even if there is one proper definition, there’s too much confusion for it to be used without further clarification, which makes it pretty useless. It’s the same idea as that thread going on right now about words so often mispronounced everyone’s given up on the right pronunciation; sometimes words just lose their meaning. It’s alright - English has got an abundance of words, we can afford to lose a couple here and there.
Well, you have the word “fortnightly,” but that’s not used in the US.
“Biweekly” to me means every two weeks. I worked as a student assistant in the Microforms and Periodicals section of the campus library as an undergrad, and the biweekly periodicals were every two weeks or even twice a month.
Here’s the Usage Note for “bi-” in the American Heritage dictionary:
The prefix bi-, when modifying an adverb of time, means that the time interval should be doubled. Thus a bimonthly meeting takes place once every two months, and a biweekly meeting once every two weeks. The prefix semi-, in the same context, means that the interval of time should be halved. Thus, a semimonthly meeting is held twice a month (that is, every half month), and a semiweekly meeting twice a week. Words with the prefix bi- and semi- are often confused, so expressions such as every two months or twice a month are often preferable in order to avoid misinterpretation.
Bi means 2 and semi means 1/2. It’s that simple. Here’s the IRS using them.
There is ambiguity because some people don’t know the correct meaning, but there is only a single correct meaning.
Like inflammable. Yeah, there might be a proper formal meaning. But if you want to be understood, you don’t say that without some clarification.
Not a good example. “Inflammable” and “flammable” have exactly the same formal meaning and are straight synonyms.
Yes, that wasn’t my point. My point was that “inflammable” isn’t a good word to use if you need to be clear. In fact, I believe that “flammible” is a back formation created precisely to alleviate this problem.
Every two weeks.
Semiweekly - twice a week
Biannual - every two years
Semiannual - twice a year
How can you possibly support both of these at the same time?
Actually, it is a good example. Because although they are synonyms, a surprising number of people don’t know that , think they are antonyms and therefore believe that “inflammable” means “not flammable”. So if you want to be certain you aren’t misunderstood, you should use “flammable”.
From years of comic book collecting, I know that “bimonthly” means every two months. If a comic you followed went from monthly to bimonthly publication, that meant it was only one step away from being cancelled.
I’m a librarian, and I happen to catalog things like magazines and journals–serials, we call them in the biz. To journal publishers, biweekly means every two weeks. Bimonthly means every two months. No exceptions. You want to indicate twice a week, the word you want is semi-weekly.
my brain is able to apply mutliple definitions to words and apply them as appropriate to a given known context.
It may not always be correct - but it works for me.
Sleeping with a guy one week and a girl the next.
“It means every two weeks” was already thoroughly covered, so I all I have left is being a wiseacre. 
I am paid biweekly. Much as I’d love that to be twice a week, it is, in fact, every other week. This is a payday week. Huzzah!
Every other week. In American English at least I don’t think I have ever heard or seen it any other way.
I thought that word was “biennial”. Biannual would be twice within a single year.
For the OP, I’d just reword the phrase so that it was less ambigious to whatever group I was addressing.
Once we solve this, can we get a definitive answer to what “next Tuesday” means? Is it the one coming up, or the one after?
tuesday - the very next one
next tuesday - the one after that, unless your on the preceeding wedneday and dealing with time zones. or maybe the very next one, depending.
‘tuesday after next’ - would be the 2nd tuesday from now, unless you’re on monday, then it might be the third one.
See, I say “this Tuesday” to mean the most immediate Tuesday coming up, and “next Tuesday” to mean the one two Tuesdays away. Some people say “next Tuesday” to mean “this Tuesday”.
My gps says " take the next left" to mean the left in front of me. That should be “this left”!
yeah - I was trying to be somewhat humourous - and completely agree -
this x - the one that is immediately in front of you -
next x - the one that is after that
x after next - the one that is after that. (2 away)
lacking a “this” then must equal the next immediate one.
My GPS must know me - it nags me to take the turn I want, and then gets all bitchy about it when it has to plan an alternate route.
maybe after that - we can discuss what ordinal we start counting at for the 3rd stoplight and how next applies to those calculations.