melodic line vs idea

melodic line and idea both are nouns.

Word	Noun	Adjective	Verb	Adverb
melodic line	Yes	No	No	No
idea	Yes	No	No	No

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

melodic line (noun)	idea (noun)
a succession of notes forming a distinctive sequence	the content of cognition; the main thing you are thinking about
	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