Basketball is a/an:

Compound Noun:
- A compound noun is a noun that is made with two or more words.
- A compound noun is usually [noun + noun] or [adjective + noun]

Examples:
- Bus-stop, Firefly, Football (Noun+Noun)
- Full moon Blackboard, Software (Adjective+Noun)
- Washing machine, Swimming Pool (Verb+Noun)
- Sunrise, Haircut, (Noun+Verb)

Likewise,
Basketball is a combination of two nouns: Basket+Ball .
So, it's a compound noun.