topic vs keynote

topic and keynote both are nouns.

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

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

Other hyponyms of topic include bone of contention, precedent, head, question.

topic (noun)	keynote (noun)
some situation or event that is thought about	a fundamental or central idea
the subject matter of a conversation or discussion	(music) the first note of a diatonic scale
	the principal theme in a speech or literary work

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