A euphemism for where an underling is required to perform sexual favors for a superior authority figure, made notable by Hollywood.
Not actually a couch, but the idea of where a person, seeking advancement, or to retain their current employment position, agree to sexual advances from their immediate superior or someone of higher authority that can insure their employment, advancement.
The term is mostly associated with the Entertainment Industry.
The term gained popularity, from various sex scandals involving major executives in the Entertainment Industry, and assorted Actors, Actresses, who claimed their position was due to their ability to provide 'sexual favors'.
The common conception being that a person would be called into the Executive Office, and would be asked to sit on a couch, where the granting of sexual favors would take place, hence the term 'casting couch'
There is no actual proof of this.
History (Legend): Rumors persist that well known Hollywood Mogul and Legendary Producer, Darryl F. Zanuck is the one who invented the 'casting couch' itself, or at least the meaning behind it.
Where one is in a position of authority over another, and requires any form of sexual involvement from someone of a lesser standing in the business operation. It can be for promotion, or for other negotiated or stipulated gains.
Practice (Associated Acts): In pornography, the casting couch is exactly that. Generally used in video production of amateur or straight solo action, a couch is used for an interview process, then the applicant is allowed to perform, usually in a solo routine.
It has been used for duo action, and is most well known for use in many 'gay for pay' pornographic productions.
It has also been a well used theme in many pornographic films, where literally, the sex takes place on a couch. Many videos use the standard concept, where one seeks sexual gratification, using their position of authority over the other.
Noteworthy: Has been made more known, due to its use in various actual shows, movies, and books, detailing the Entertainment Industry. Popular sitcom shows such as Friends have had episodes dealing with the casting couch.