sesquiannual adjective1 •Occurring once every one and a half years (i.e. once every 18 months).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, page 718) — 1979
