idea vs underrating

idea and underrating both are nouns.

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

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

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

idea (noun)	underrating (noun)
the content of cognition; the main thing you are thinking about	an estimation that is too low; an estimate that is less than the true or actual value
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