Here's why:
* Two words combined: It's formed by combining two words, "football" and "stadium", to create a new meaning.
* Single concept: It refers to a single entity, a stadium specifically designed for playing football.
* Usually written together: While it can be written as two separate words, it's more common to see it hyphenated or written as one word.
So, "football stadium" is a compound noun because it combines two words to create a single, new meaning.