It's most commonly used as an interjection, which is a word that expresses emotion or feeling (like "wow!" or "ouch!").
Here's why it's not a noun:
* It doesn't name a person, place, or thing. Nouns are words that identify something.
* It doesn't function grammatically like a noun. Nouns can be the subject of a sentence, the object of a verb, or the object of a preposition. "Please" doesn't fit those roles.
Example:
* Incorrect: "I need a please." (This makes no sense because "please" doesn't refer to a thing you can need.)
* Correct: "Could you please pass the salt?" ("Please" is an interjection that politely requests something.)