idea vs melodic theme

idea and melodic theme both are nouns.

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

As nouns, idea and melodic theme are synonyms defined as:

idea and melodic theme: (music) melodic subject of a musical composition

Other synonyms of idea include musical theme, theme.

idea (noun)	melodic theme (noun)
the content of cognition; the main thing you are thinking about	(music) melodic subject of a musical composition
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