Why is the word criterion irregular?

To understand irregular nouns, you need to know what a regular noun is. A regular noun is a noun that forms its plural by adding an 's' or an 'es' to the end of the Word. An irregular nounis a noun that forms its plural in some other way.

The noun 'criterion' forms its plural in the old Greek way: one criterion, two criteria.

Other examples are phenomenon-phenomema, medium-media, or curriculum-curricula.