Fortnightly is how I would express every 14 days.

Americans don't generally use that word.

For some reason the bi-weekly vs. semi-weekly thing doesn't confuse me, but I take your point and the dictionary seems to agree that it can mean either every two weeks or twice a week.

It confuses me because I regularly hear "bi-weekly" to mean "twice per week". I don't think I've ever heard someone say semi-weekly. Usually you can make a good guess of the intended meaning from context, but the two time intervals are similar enough that it's frequently ambiguous.

I guess I’m just generalizing semi-weekly from semi-annual. Do people actual my use the term bi-weekly to mean twice a week? I understand confusing bi-annual to mean twice a year because years can be evenly split in two and bi-annual would thus mean every six months. But weeks, being 7 days long, don’t naturally get cut in half. If you heard someone use the phrase bi-weekly and the context implied twice a week, what would that even mean to you? It seems to me that someone would always say the exact days of the week they meant.

Interesting... in my area (NYC) I only hear people say it to mean 14 days. I suppose the rationale is to think of "bi" as "two" (precisely as you'd do for bicycle, bisexual, binomial, etc.) rather than as "twice" and then the ambiguity disappears. Counterexample supporting ambiguity: the word bisect, which means two sections, but can quite easily also be thought of as halving!

A lot of people dictionary words incorrectly and the dictionary unfortunately adapts and decays the language into incomprehensibility. That doesn't make correct usage wrong.

fort-weekly

The 'fort' is short for fourteen, so fort-weekly would be every fourteen weeks.

fortnightly

Heh, I just realized that this means that biweekly (once per two weeks) is about the same as bimonthly (twice per month)

Biweekly is twice per week just like biannually is twice per anum.

Fortnightly and biennially are 'every other'.

The fact that the parent comment used Biweekly to mean 14 days seems to suggest that it's not unambiguous.

It’s ambiguous if people misunderstand you even if they’re “wrong” for being confused. Avoid those terms. People will widely misinterpret them.

A bicycle is not half a wheel, it is two wheels. If bi-annual is twice a year, then why does the word semi-annual exist? And I’m American, we don’t use the word fortnight unless we’re pretending to be British.

> then why does the word semi-annual exist

My large Collins dictionary suggests that biannual means twice per year, while semi-annual has more of a hint at 'six-monthly', i.e. evenly space (which makes sense if you think semi-circle, semi-detached, etc.).

> And I’m American, we don’t use the word fortnight unless we’re pretending to be British.

Ah well there's the problem! ;) But well, yes, of course if you remove the word for something from your vocabulary there's going to be ambiguity when a similar one is used. Or just stick to 'every [other] week' and remove biweekly too.

"why does the word semi-annual exist"

Your parent already provided the counterpoint, Biennially.

Additionally I'm not convinced by your bicycle => 2 => bi annual logic.

You could equally argue it leads to 2 times a year, because it doesn't happen a half every year just like a bike doesn't have half a wheel.

Mostly I'm just arguing that it's ambiguous. I don't have a strong opinion, but the person I was responding to did and it seems like a silly thing to be so adamant about.

I'm 'so adamant' about what they mean only in the same sense that I am about any other word I know the meaning of, I only meant to be helpful.

For your bicycle example that I didn't reply to initially - I don't see what it breaks? Unicycles have one and tricycles three wheels. If i give you a wheel triweekly you have three by the end of the week and can build a tricycle.

FWIW, regarding correct usage: the dictionary on my computer (Oxford American English) lists both meanings of biweekly and does not put either meaning forward as more correct. It even has a whole entry on the ambiguity of the 'bi' prefix.

I can confirm that the proper OED has the same definition.

"Bi- 2a occurring twice in every one or once in every two (biweekly)"

Biannual is only defined as occurring twice a year though.

So, inflammable.

That's wrong in multiple ways. Fortnightly is 14, and bi is ambiguous because English has weak grammatical structure inside words.

Yes, 14 days (nights), which is - as I phrased it - every other week.

I didn't claim any strong grammatical structure of 'bi', just consistency between biweekly and biannually, and that 'once per two weeks' was the meaning of a different word, as is 'once per two years'.