from
The Collaborative International Dictionary of English v.0.48
Music drama \Mu"sic dra`ma\
An opera in which the text and action are not interrupted by
set arias, duets, etc., the music being determined throughout
by dramatic appropriateness; musical drama of this character,
in general. It involves the use of a kind of melodious
declamation, the development of leitmotif, great orchestral
elaboration, and a fusion of poetry, music, action, and scene
into an organic whole. The term is applied esp. to the later
works of Wagner: "Tristan und Isolde," "Die Meistersinger,"
"Rheingold," "Walk["u]re," "Siegfried,"
"G["o]tterd[aum]mmerung," and "Parsifal."
[Webster 1913 Suppl.]