Source: It’s the terminology banks use when discussing paychecks. If you get paid on the same days every month (like the 1st and the 15th) then you get paid bimonthly. If you get paid every two weeks regardless of the date, (like every other Friday) then you get paid semiweekly.
Interesting, I've come to understand those prefixes to have the opposite meaning. I looked into it, and apparently the actual usage of bi vs semi is not super consistent, and has been historically ambiguous and especially difficult because of how confidently people tend to believe others share their understanding on the words.
You'll notice a lot of the words you listed include both "twice per" and "once every other" in their dictionary definitions. Of course, dictionaries are descriptive and not prescriptive indicators of language, and I think it's fine to argue one's case to keep the terminology consistent at least within the bounds of a conversation.
I guess the takeaway is that it's good to have a context under which the terms are consistent, like with banking, but also important to understand that there are other contexts with definitions that differ, and that those different contexts aren't incorrect.
"Semi" literally means "half." A semi-annual event occurs every half year, i.e. twice each year. A semicircle is half of a circle; there are two to a full circle. Of this one, I am certain.
"Bi" typically means "two" or "twice." Its usage is highly inconsistent in terms of frequency. I think its proper usage would mean "every other" (i.e. every two), but I don't know for sure. Either way, the inconsistency of its usage makes it ambiguous.
For the most part, confusion about usage comes in with monthly or weekly events. It sounds awkward to use "semi" in these cases -- honestly, who can say "semi-weekly" without sounding like a prick? -- so "bi" is typically used, thus exacerbating confusion.
Of course, I sound like a prick through all of this comment, so what can I say?
Something in my ape brain just immediately trusts anyone who uses the phrase "of this one, I am certain." So no, you don't sound like a prick, you sound like a scholar, and I am inclined to request that you make several of my financial decisions for me.
You are right. A biweekly meeting is every other week. Bi also just means two. Look at bicycle... yeah English pronunciation fucks it up but it literally means two wheels.
Bisexual and pansexual are indeed interchangeable. But if someone claims to be pansexual, because they specifically are interested in dating trans people... Well they might have some gender questioning at some point.