Constitutionalism is when the person in charge of a country, say a monarch or president, is limited in power and the things that he can do. A constitution can be a written document or it can be a known fact.
A Jesuit Spaniard, Juan de Mariana wrote a couple of books on limiting the power of monarchs such as De Rege et Regis Institutione (On the King and the Royal Institution). He stated in his book that if the monarch doesn’t abide by the constitution or he’s ruling like a tyrant he should be assassinated. In one of his other books, De Monetae Mutatione (On the Alteration of Money), Juan wrote about how it was not just for a ruler to inflate money (alter money). This book got Juan arrested for a period of time because the king of Spain did not appreciate him talking down about money inflation. Although what’s odd is that it didn’t seem that Louis had an issue with Juan suggesting that if a king is a tyrant he should he assassinated.
Thanks to constitutionalism we can identify when a government official is overstepping his bounds!