That's what it means etymologically: "six months," but then its meaning changed or became more specific like so many other words have. In Spanish we still pretty much keep the original Latin meanigs:
Bimestre = (a) two months (period)
Trimestre = three months.
Cuatrimestre = four months
Semestre = six months.
And depending on the country and university, university terms usually have a duration of either four months (cuatrimestres) or six (semestres), so their names still manage to reflect their etymology. It's funny how it came to mean "vacation" in Swedish, though.
Speaking of false friends, here's an extensive list: http://sv.wikipedia.org/wiki/Lista_%C3%B6ver_falska_v%C3%A4nner_mellan_svenska_och_engelska
Where I'm from just south of London, we'd definitely go for "six months" over "half of a year". "Half of a year" is a strange construction, and we'd probably use "half a year" instead, but we'd definitely err towards "six months" even over that, eg: "It'll take six months" vs "It'll take half a year". I think it's partly out of laziness—"six months" is a whole syllable shorter than "half a year" and two whole syllables shorter than "half of a year", which as I mentioned above just sounds odd—and partly because of a general tendency to switch to larger units only when you pass the minimum threshold for the next unit up, eg:
- 6 days
- a week
- four weeks
- one month
- six months
- 11 months
- a year
- a year and a day
- a year and three weeks
- a year-and-a-quarter (I really should find out exactly why it is that I hyphenate this…)
- a year-and-a-half (and this)
That said, we do say "six weeks" and "12 weeks" instead of "a month and a half" and "three months", I suspect because "six weeks" is three whole syllables shorter than "a month and a half", and "12 weeks" because we're English and we like counting things in twelves.
Yeah, laziness is definitely a common reason for why we do things the way we do in English. o.o
"Half a year" is a lot easier to say than "six months" though!
I'm gonna disagree and say they're pretty much interchangeable, except when you're being precise it makes sense to use the smaller unit - half a year could easily mean a month either way, six months sounds like you're being more specific. Twenty-six weeks feels like an even more solid, businesslike definition, and so on. Otherwise people seem to just go with whichever they feel like, I haven't noticed any trend either way personally
Well, if you see it in Latin, it refers to the period of six months (SEX + MENSIS). In Portuguese it is also the period of six months, regardless of institution. Most definitions I see in English, though, refer to the "college period" (e.g. http://www.oxfordlearnersdictionaries.com/us/definition/english/semester?q=semester). That's why I brought the subject
If you refer to trimester, it is the period of three months: http://www.oxfordlearnersdictionaries.com/us/definition/english/trimester