The first "nacho cheese" was cheddar (back when nachos were purportedly invented by Ignacio Anaya in 1943), but ever since bottled and canned varieties hit shelves, the recipe has been in a constant state of flux. What we do know is that it's usually spicy, salty, and totally not just cheddar. In fact, Old El Paso's recipe contains both cheddar and blue cheese, while Pringles' uses cheddar, Romano, and Parmesan.
So, as far as nacho cheese flavorings go, it looks like -- no matter which type of chip you go with -- it's a bit of a mixed bag.
Adam Lapetina is a Food/Drink staff writer for Thrillist, and doesn't need a definition to know that nacho cheese is delicious. Read his musings at @adamlapetina.