opera vs musical drama

opera and musical drama both are nouns.

Word	Noun	Adjective	Verb	Adverb
opera	Yes	No	No	No
musical drama	Yes	No	No	No

As nouns, musical drama is a hyponym of opera; that is, musical drama is a word with a more specific, narrower meaning than opera:

opera: a drama set to music; consists of singing with orchestral accompaniment and an orchestral overture and interludes
musical drama: opera in which the musical and dramatic elements are equally important; the music is appropriate to the action

Other hyponyms of opera include rock opera, bouffe, comic opera, opera bouffe, opera comique, grand opera.

opera (noun)	musical drama (noun)
a building where musical dramas are performed	opera in which the musical and dramatic elements are equally important; the music is appropriate to the action
a drama set to music; consists of singing with orchestral accompaniment and an orchestral overture and interludes