Why is the word criterion irregular?

The word "criterion" is considered irregular because it doesn't follow the typical pattern of forming plurals in English.

Here's why:

* Most nouns in English form their plural by adding -s or -es: For example, "cat" becomes "cats," and "box" becomes "boxes."

* "Criterion" is a noun of Greek origin: It comes from the Greek word "kritērion," meaning "standard of judgment."

* Greek plurals often end in -a: For example, "phenomenon" becomes "phenomena."

Therefore, the plural of "criterion" is criteria. This is a direct adoption of the Greek plural form, not following the regular English pluralization rules.