Obviously every year is annual. Every two years is biennial. Does the English language have a term for every 18 months?
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
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-'.
protected by RegDwigнt♦ Aug 2 '12 at 16:39
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