It refers to a complex religious tradition that originated in the Indian subcontinent.
Here's why it's not other parts of speech:
* Not a verb: You don't "hinduism" something.
* Not an adjective: You wouldn't describe something as "hinduism." You might say "Hindu" or "Hinduistic."
* Not an adverb: You don't do something "hinduismly."
Let me know if you have any other questions about Hinduism or grammar!