A backroom is a room that is situated at the back of a building, especially a private room.

You can use backroom to refer to people in an organization who do important work but are not seen or known about by the public. You can also use backroom to refer to a place where such people work.

If you refer to a deal made by someone such as a politician as a backroom deal, you disapprove of it because it has been made in a secret, dishonest way.