What is the meaning of Music drama?

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 especially to the later works of Wagner: "Tristan und Isolde," "Die Meistersinger," "Rheingold," "Walkure," "Siegfried," "Gotterdammerung," and "Parsifal.".