idea vs opinion

idea and opinion both are nouns.

Word	Noun	Adjective	Verb	Adverb
idea	Yes	No	No	No
opinion	Yes	No	No	No

As nouns, opinion is a hyponym of idea; that is, opinion is a word with a more specific, narrower meaning than idea:

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

idea: a personal view
opinion: a personal belief or judgment that is not founded on proof or certainty

Other hypernyms of idea include persuasion, sentiment, thought, view.

idea (noun)	opinion (noun)
the content of cognition; the main thing you are thinking about	the reason for a court's judgment (as opposed to the decision itself)
your intention; what you intend to do	a personal belief or judgment that is not founded on proof or certainty
an approximate calculation of quantity or degree or worth	a belief or sentiment shared by most people; the voice of the people
a personal view	a vague idea in which some confidence is placed
(music) melodic subject of a musical composition	the legal document stating the reasons for a judicial decision
	a message expressing a belief about something; the expression of a belief that is held with confidence but not substantiated by positive knowledge or proof