Here's why:
* It names a person (or deity): Nouns are words that name people, places, things, or ideas. Zeus is a specific name.
* It can be the subject of a sentence: "Zeus ruled the heavens." Here, "Zeus" is the subject of the sentence.
* It can be the object of a verb: "Hera worshipped Zeus." In this sentence, "Zeus" is the object of the verb "worshipped".
Therefore, "Zeus" functions as a noun in the English language.