Sesquiannual, which means occurring every one and a half years, is a word that is rarely used. Hence, it can be challenging to find words that have the same meaning. A few potential substitutes for sesquiannual include "18-month periodical" or "year and a half recurring." Another option could be to use the mathematical term "one and a half-year cycle." Saying something occurs every two years and four months could also convey the same meaning. In summary, while there may not be many conventional synonyms for sesquiannual, there are alternative ways to express the same idea.