a form of opera, specif. as developed by Richard Wagner, characterized by a continuous flow of orchestral music, with an integrative use of musical themes (leitmotifs), and singing that is free from formal division into arias, recitatives, etc.
See music drama in American Heritage Dictionary 4
noun
An opera that avoids discrete numbers such as arias, recitatives, or ensembles, and in which the music reflects or embodies the action of the drama.