In the Middle Ages the region was called Abruzzo, from the Latin Aprutium. Later, the region was divided into three areas, Abruzzo Citeriore, Abruzzo Ulteriore I and Abruzzo Ulteriore II. So if you’ve taken Italian you know that the plural of a word ending in an “o” ends in “i” – thus Abruzzi referred to all three of these areas.
When Italy unifed in 1860, the region of Molise was joined to these three areas and the region came to be known as Abruzzi e Molise. In 1963, the Italian government separated Abruzzo and Molise into two distinct regions. Italy has a total of 20 regions in all.
I can now sleep better at night knowing the explantion. Hope you can too!