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Obviously every year is annual. Every two years is biennial. Does the English language have a term for every 18 months?

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What's the point of using that word if almost no one knows the meaning though =) – Andreas Bonini Jun 7 '11 at 14:51
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@Kop to sound clever ;) I guess as @JoseK says it's interesting to introduce a new word once in a while. – planetjones Jun 7 '11 at 15:10

2 Answers

up vote 48 down vote accepted

sesquiennial

ocurring every year and a half.

Etymology: semi ("half") + que ("and")

I cannot find a dictionary definition of this other than Wiktionary, and 0 hits on Google NGram.

But there are some examples of usage here

FAME now has responsibility for the sesquiennial (every 18 months) Music Festivals which attract players from Europe, American continent

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thank you JoseK - although my girlfriend is now debating whether it's an acceptable phrase to use in a document at work! – planetjones Jun 7 '11 at 10:47
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@planetjones: Well, he answered to your question. If are not comfortable using "sesquiennial", just use "every 18 months", it's not that bad. :D @JoseK: Where did you find it? :D ahah +1 – Alenanno Jun 7 '11 at 10:48
@planetjones: Well, I only learnt this while researching your answer - dictionary.com asks "Did you mean sexennial?" I'll say this will have people searching in the dictionary for the meaning - so they'll have learnt something too. – JoseK Jun 7 '11 at 10:52
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@Alennano: I picked up sesquicentennial (which means 150 years) listed on Wikipedia and did a guess search for sesquiennial – JoseK Jun 7 '11 at 10:55
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@Rei Miyasaka I'm not sure sesquiennial is a clear example of the "gobbledygook, jargon and misleading public information" which the plain English campaign states they are against. – planetjones Jun 8 '11 at 8:50
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up vote 11 down vote

I think JoseK's answer is the correct one, but there are also a few google hits for 'semi-triannual' and even a few for the (more correct, because unambiguous) 'semi-triennial.' Either of these might be easier for casual readers to decipher than 'sesquiennial,' since 'semi-' and 'tri-' are more familiar than 'sesqui-'.

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The root is not "sesqui-" but "se" (which comes from semi-). "qui" is a "conjuction". – Alenanno Jun 7 '11 at 14:54
@Alenanno, I'm not talking about roots, I'm talking about English-language prefixes. 'sesqui-' may not be a single root, but it is a single prefix; it's in the dictionary. – senderle Jun 7 '11 at 15:47
I was talking about the original term. :) – Alenanno Jun 7 '11 at 15:50
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+1 not because it's the 'right' answer, but because semi-triennial is potentially more comprehensible to millions of people on first encounter than sesquiennial. I find it somewhat ironic that @Josek's only quoted usage example feels the need to define the term immediately after writing it! :-) – FumbleFingers Jun 7 '11 at 17:24

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