Noun
(uncountable) A genre of opera written to be performed with a chamber ensemble.
(countable) An individual opera in this genre.
Source: en.wiktionary.orgBarnett, p. 106 This is the Rill Speaking is a one-act chamber opera that Wilson adapted from his own play of the same name. Source: Internet
Holst (1969), pp. 51–52 He continued to teach and compose; he worked on The Planets and prepared his chamber opera Savitri for performance. Source: Internet
The Vancouver Opera is a major opera company in the city, and City Opera of Vancouver is the city's professional chamber opera company. Source: Internet