idea vs notion

idea and notion both are nouns.

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

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

idea (noun)	notion (noun)
the content of cognition; the main thing you are thinking about	(usually plural) small personal articles or clothing or sewing items
your intention; what you intend to do	a general inclusive concept
an approximate calculation of quantity or degree or worth	an odd or fanciful or capricious idea
a personal view	a vague idea in which some confidence is placed
(music) melodic subject of a musical composition