idea vs whimsy

idea and whimsy both are nouns.

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

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

idea (noun)	whimsy (noun)
the content of cognition; the main thing you are thinking about	the trait of acting unpredictably and more from whim or caprice than from reason or judgment
your intention; what you intend to do	an odd or fanciful or capricious idea
an approximate calculation of quantity or degree or worth
a personal view
(music) melodic subject of a musical composition