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Half a year is a semester, i.e. (literally) 6 months. Since it’s often wrongly thought to derive from semi- ‘half’, there’re contradicting definitions of similar terms: Both a trimester and a (rare) quadmester are either 3 months, i.e. a fourth of a year, or a third of a year, i.e. 4 months. The definitions are mutually exclusive, that means the terms never mean the same duration in the same context. These are academic terms that usually subdivide the academic year from autumn to autumn, not the calendar year. Human pregnancies are also often divided into 3 three-month trimesters.

In economics and accounting, a quarter or quarter-year is often used, which is either defined as 3 months or as 13 weeks, i.e. 90–92 days. The possible alternative term fourth is not being used, as far as I know.

What would you call a sixth part of the calendar year, i.e. 2 collective months or about 60 days?

From other European languages, I’d assume sextal, but the OED only lists ‘base 6’ as a meaning of that word (like ‘octal’) and it’s not quartal (or tertial and quintal) in English but quarter – alas, sexter seems just wrong. Sometimes the Romance x becomes an s, and sester used to be a (Middle) English measure indeed. I could also go Greek (hexa-) instead of Latin.

I guess double-month or di-/bi-month could also work. A neologism is fine as long as it’s understandable and doesn’t raise wrong associations like a six-month span.

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3 (consecutive) (calendar) months can equate to 89 days. Are you saying that 'quarter' is only used with Jan-Feb-Mar etc, or can be used with any 3-month period? – Edwin Ashworth Mar 5 at 15:20
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@EdwinAshworth In my experience, quarter usually subdivides the calendar year, but it’s actually quite possible that it’s also used to partition the tax or fiscal year, which in some legislations can begin any day of the year (and sometimes have just 360 nominal days). Trimesters or quadmesters and seasons, on the other hand, are almost never Jan-Feb-Mar, Apr-May-Jun, Jul-Aug-Sep and Oct-Nov-Dec (or 13-week approximations thereof). – Crissov Mar 5 at 15:42
    
Sometimes two months is just two months. – Hot Licks Mar 5 at 21:19
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IN UK English I've never heard of a word used in that context. However, when a magazine is published once every 2 months, it is referred to as a bi-monthly publication. That's the only similar language usage I can think of. – David Wilson Mar 6 at 0:43
    
I'd be careful about a semester being exactly six months, in colleges it's about four, usually with time off in the summer. It would be very confusing to use semester to refer to a six month period outside the usual semesters beginning in September and January, to an academic audience at least. – Azor-Ahai Mar 8 at 19:45
up vote 14 down vote accepted

I think you are looking for the term bimester:

  • a period of two months

(M-W)

Usage examples:

In each case they contitute an average of the responses in the October- November bimester. (Use of Survey Data for Industry, 1999.)

We proposed a bimester alternative to the trimester model. (Undergraduate Projects Linking Science, Technology and Society. )

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Yes, it works, but I would prefer a term for a sixth of a year over one for two months, although that will often be the same. – Crissov Mar 5 at 15:55
    
@Crissov Are you talking about non-Gregorian calendars, or is there a risk of ambiguity because members of your audience might not adhere to 12-month years? Or is the question of temporal alignment, for example from mid-March to mid-May is a sixth of a year but spans 3 calendar months (though its duration is shorter than any 3 calendar months)? But, then, all your other examples (quarter, trimester, etc) align to calendar month boundaries. Otherwise, when can a sixth of a year differ from two calendar months? Planets with other periods of revolution? – Dan Bron Mar 5 at 17:22
    
@DanBron I’m indeed talking about other calendars, but the Gregorian calendar is of course universally known and thus can serve as a reference. A sixth of a year (almost 61 days) is within the range of lengths of two consecutive months (59 to 62 days), of course, but in my case, the sixths are rounded to full weeks (56 or 63 days). I thought that was a bit too exotic and specific to be put into the question. – Crissov Mar 5 at 17:44
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@Crissov: well, if "semester" can be used to refer to half a year even when that doesn't mean six exact months, then "bimester" by analogy could refer to a sixth of a year even when that is not exactly two months. – sumelic Mar 5 at 21:22
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This is a 90s term that never caught on. Ngram of bimester and half of a quarter – Mazura Mar 5 at 22:39

According to Wikipedia, the numerical prefix corresponding to "sixth" is sextant- (which seems to be derived from the Latin word for sixth, sextans). In fact, I can't find any English word that uses this as a prefix.

But if this is accurate, it suggests the neologism sextantennium (sixth of a year) by analogy with words like millennium (thousand year period) and sesquiennium/sesquiennial (a period of one and a half years/every one and a half years, also mentioned here: Is there a term to describe an event which happens every 18 months?).

I searched for sextantennium on Google and found zero results. Personally, I'd go with Josh61's suggestion of bimester.

Another related question:

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In some cases, the simplest solution is "60 days". Or "two months".

(I've never quite understood why we get so many questions seeking single commonly known words for concepts. There is no rule saying such words have to exist, and there is nothing wrong with using a short phrase as, effectively, a compound word. Yes, seeking the perfect word is a fun game, but unless you already know it or are writing poetry, finding it is largely a distraction from actually communicating... And often the best way to find out if simply to "consult the oracular lizard of the ancient lexophile culture" and ask a thesaurus.)

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