sesquiennial

Adjective

sesquiennial (not comparable)

1. Occurring once every one and a half years (i.e. once every 18 months, or twice every three years); sesquiannual.
   • The endemic Australian sea lion is one of the world's rarest and most unusual seals: rare in terms of very small numbers and unusual in its having a sesquiennial reproductive cycle.
     [Australian sea lions' breeding cycle is a 17-18 month aseasonal one.]2009, John K. Ling, Australian Sea Lion, Neophoca cinerea, in the Encyclopedia of Marine Mammals →ISBN
   • Delta Mu Delta, a national honor society in business administration, held its regular sesquiennial meeting at New York Institute of Technology DeSeversky Conference Center in Old Westbury, Long Island, New York, on Saturday, April 24, 1976. […]
   • The next sesquiennial meeting of Delta Mu Delta is scheduled for October, 1977. […]
   • The regular sesquiennial meeting of the National Chapter of Delta Mu Delta national honor society in business administration will be held on Saturday, October 29, 1977, at Bucknell University in Lewisburg, Pennsylvania.1977, CN & V, volumes 27-30, pages 39–40