sesquiennial

Etymology
From (compare  and ).

Adjective

 * 1) Occurring once every one and a half years (i.e. once every 18 months, or twice every three years); sesquiannual.
 * 2) * 2009, John K. Ling, Australian Sea Lion, Neophoca cinerea, in the Encyclopedia of Marine Mammals ISBN 0080919936:
 * 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.]