idea vs preoccupation

idea and preoccupation both are nouns.

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

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

idea (noun)	preoccupation (noun)
the content of cognition; the main thing you are thinking about	the act of taking occupancy before someone else does
your intention; what you intend to do	the mental state of being preoccupied by something
an approximate calculation of quantity or degree or worth	an idea that preoccupies the mind and holds the attention
a personal view
(music) melodic subject of a musical composition