sesquiannual

Adjective

 * 1) Occurring once every one and a half years (i.e. once every 18 months).
 * 2) * 1979, R. E. Meyer, Sesquiannual Report – April 1977-September 1978, Fossil Energy Division/DOE (DOE/W-7405-eng–26-1; cited in Surface Phenomena in Enhanced Oil Recovery (2013, ISBN 1475703376, page 718)
 * 3) * 1990, Sesquiannual Meeting of the American Society for Rickettsiology and Rickettsial Diseases; 8 (the society holds meetings roughly every 1.5 years, e.g. September 1989, May 1991, and September 1992)

Usage notes
Conventionally, X-annual terms (like biannual) mean "X number of times a year" and so this term would be expected to mean "one and a half times a year, i.e. once every 8 months"; however, that sense is not attested; in use, this term is treated as a synonym of.