idea vs impression

idea and impression both are nouns.

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

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

idea (noun)	impression (noun)
the content of cognition; the main thing you are thinking about	an impressionistic portrayal of a person
your intention; what you intend to do	the act of pressing one thing on or into the surface of another
an approximate calculation of quantity or degree or worth	(dentistry) an imprint of the teeth and gums in wax or plaster
a personal view	an outward appearance
(music) melodic subject of a musical composition	a vague idea in which some confidence is placed
	a clear and telling mental image
	a symbol that is the result of printing or engraving
	all the copies of a work printed at one time
	a concavity in a surface produced by pressing