opinion poll vs inquiry

opinion poll and inquiry both are nouns.

Word	Noun	Adjective	Verb	Adverb
opinion poll	Yes	No	No	No
inquiry	Yes	No	No	No

As nouns, inquiry is a hypernym of opinion poll; that is, inquiry is a word with a broader meaning than opinion poll:

opinion poll: an inquiry into public opinion conducted by interviewing a random sample of people
inquiry: a search for knowledge

Other hypernyms of opinion poll include enquiry, research.

opinion poll (noun)	inquiry (noun)
an inquiry into public opinion conducted by interviewing a random sample of people	a systematic investigation of a matter of public interest
	a search for knowledge
	an instance of questioning