leitmotiv vs melodic line

leitmotiv and melodic line both are nouns.

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

As nouns, melodic line is a hypernym of leitmotiv; that is, melodic line is a word with a broader meaning than leitmotiv:

leitmotiv: a melodic phrase that accompanies the reappearance of a person or situation (as in Wagner's operas)
melodic line: a succession of notes forming a distinctive sequence

Other hypernyms of leitmotiv include air, line, melodic phrase, melody, strain, tune.

leitmotiv (noun)	melodic line (noun)
a melodic phrase that accompanies the reappearance of a person or situation (as in Wagner's operas)	a succession of notes forming a distinctive sequence