underrating vs idea

underrating and idea both are nouns.

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

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

underrating: an estimation that is too low; an estimate that is less than the true or actual value
idea: an approximate calculation of quantity or degree or worth

Other hypernyms of underrating include approximation, estimate, estimation.

underrating (noun)	idea (noun)
an estimation that is too low; an estimate that is less than the true or actual value	the content of cognition; the main thing you are thinking about
	your intention; what you intend to do
	an approximate calculation of quantity or degree or worth
	a personal view
	(music) melodic subject of a musical composition