The “biweekly” word is difficult. Does it mean “twice per week” or “every two weeks”? It’s an old chestnut, and generations have looked for an answer. I mustn’t call it the definition of ambiguity, because that’s not what it is, but it is a good example of it.
I gave some thought to the thing. Since we’ve already got a perfectly good word for “every two weeks” (“fortnightly”), I decided that “biweekly” must be used for all other situations, and to make that my recommendation. I felt jolly pleased with myself, and rightly so, right?
Well, no. Here’s Marco Arment in a footnote to a blog post about a podcast about sodding market analysis (my sentiment, not his, although I’ll admit that I listen to it too. Anyhow, it’s a fortnightly podcast):
By convention, “biweekly” should mean “every two weeks”. Unfortunately, the bi- suffix has been misunderstood so frequently to mean “twice per” instead of “every two” that “biweekly” now uselessly means “every two weeks” or “twice per week”.
The semi- prefix properly, and only, means “twice per”. Therefore, the correct word for “twice per week” is “semiweekly”. But since misunderstandings of this have ruined bi-, there’s now no English word that unambiguously means “every two weeks”. (Except “fortnightly”, which is ridiculous.)
I’m using “biweekly” to properly mean “every two weeks” in protest of this widespread definition-shifting vocabularly ignorance.