Initially coined by Theodor Mundt in 1833, it was most notably used by Richard Wagner, along with Gesamtkunstwerk, to define his operas.
Mundt formulated his definition explicitly in contrast to intermezzo, or a piece that sits in between dramatic entities.
To this day, Musikdrama is associated with the works of Richard Wagner where poetry, music and stage performances were not arbitrarily combined.
Wagner himself resisted calling his works Musikdrama, which would imply a drama "meant for music," like a libretto.
A major characteristic of Musikdrama is its formal unity, without interruptions or smaller closed forms such as arias or duets.