idea vs keynote

idea and keynote both are nouns.

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

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

idea (noun)	keynote (noun)
the content of cognition; the main thing you are thinking about	a fundamental or central idea
your intention; what you intend to do	(music) the first note of a diatonic scale
an approximate calculation of quantity or degree or worth	the principal theme in a speech or literary work
a personal view
(music) melodic subject of a musical composition

idea (verb)	keynote (verb)
	set the keynote of
	give the keynote address to (an audience)