Most theatrical productions have some type of scenerywalls, arches, painted backdrops, and other background forms. The visual surroundings on stage that represent the setting of the play are called the set. This photo shows the set for the Lexington Childrens Theatre production of Theres a Boy in the Girls Bathroom.
(FYI: The Greek playwright Sophocles is believed to have been one of the first to use scene painting as stage decoration. Almost 2,000 years later, in the 1600s in Renaissance Italy, elaborate stage sets were designed and built and became almost as important to audiences as the actors themselves.)