It is often described as an electronic opera, or electroacoustic oratorio.

Described as a comic opera, it has nevertheless been noted to contain a serious theme.

He described the work as a "lyric tragedy" rather than an opera, since there is very little action.

Handler has described it as an "opera in book form".

Nightingale (1982; this work is often described as an opera)

The film is described as a hip-hop opera set in the criminal justice system.

Christus was described by its composer as a 'sacred opera'.

Smith also described it as "less an opera in any conventional sense than a multimedia monodrama".

Messiaen preferred to describe the final work as a "spectacle" rather than an opera.

The composer himself described the work as a 'Morality' rather than an opera.