leitmotif vs melodic line

leitmotif and melodic line both are nouns.

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

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

leitmotif: 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 leitmotif include air, line, melodic phrase, melody, strain, tune.

leitmotif (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