(Latin: one and a half; normally used as a prefix; from Latin, semis “half” + que “and”)

In chemistry, three atoms or equivalents of the [specified] element or radical are combined with two of another, as in iron sesquioxide, Fe 2O3. Also defined as the proportion of two (of one radical or element) to three (of another).

Remember: You can enjoy seeing many examples of sesquipedalianisms with solutions, starting with Group 1-17, by going to sesquipedalia verba.