type of serious musical theatre, first advanced by Richard Wagner (Wagner, Richard) in his book Oper und Drama (1850–51; “Opera and Drama”), that was originally referred to as simply “drama.” (Wagner himself never used the term music drama, which was later used by his successors and by critics and scholars.) This new type of work was intended as a return to the Greek drama as Wagner understood it—the public expression of national human aspirations in symbolic form by enacting racial myths and using music for the full expression of the dramatic action. Wagner's emphasis on opera as drama merely resumed and developed the ideas of Claudio Monteverdi and Christoph Gluck. He envisaged the disappearance of the old type of opera, with its libretto provided by a hack versifier, as an opportunity for the composer to make a “set piece” opera out of purely musical forms separated by a recitative.

Briefly put, the new art form would be created by a single artist, who would write a poetic drama that should find full expression when it was set to a continuous vocal-symphonic texture. This texture would be woven from basic thematic ideas, or leitmotivs (leitmotiv) (“leading motives”); these would arise naturally as expressive vocal phrases sung by characters at crucial emotional points of the drama and then be developed by the orchestra as “reminiscences” in accordance with the expressive need of the dramatic and psychological development of the action. This conception found full embodiment in Der Ring des Nibelungen, a cycle of four operas first performed in 1876; the only variation from Wagner's theory was that the leading motives did not always arise as vocal utterances but were often introduced by the orchestra to portray characters, emotions, or events.