theme song vs melody

theme song and melody both are nouns.

Word	Noun	Adjective	Verb	Adverb
theme song	Yes	No	No	No
melody	Yes	No	No	No

As nouns, melody is a hypernym of theme song; that is, melody is a word with a broader meaning than theme song:

theme song: a melody that recurs and comes to represent a musical play or movie
melody: a succession of notes forming a distinctive sequence

Other hypernyms of theme song include air, line, melodic line, melodic phrase, strain, tune.

As nouns, melody is a hypernym of theme song; that is, melody is a word with a broader meaning than theme song:

theme song: a melody used to identify a performer or a dance band or radio/tv program
melody: a succession of notes forming a distinctive sequence

Other hypernyms of theme song include air, line, melodic line, melodic phrase, strain, tune.

theme song (noun)	melody (noun)
a melody that recurs and comes to represent a musical play or movie	the perception of pleasant arrangements of musical notes
a melody used to identify a performer or a dance band or radio/tv program	a succession of notes forming a distinctive sequence