Articles are a type of determiner, which are words that come before nouns and specify which noun is being referred to.
Here's why it's not an adjective or noun:
* Adjectives describe nouns. For example, "red" is an adjective that describes a noun like "ball".
* Nouns are words that name people, places, things, or ideas. For example, "ball" is a noun.
"An" doesn't describe a noun or name anything. It's a word that helps us understand which noun is being referred to, like in "an apple" or "an idea".