Which term refers to the side areas of a stage that cannot be seen by the audience?

The term that refers to the side areas of a stage that cannot be seen by the audience is "wings." Wings are the offstage areas located on either side of the acting area, where actors can wait to make their entrances, and where set pieces and props are stored out of sight. This practice allows for seamless transitions between scenes and helps maintain the illusion of the performance for the audience.

Front of house typically refers to areas where the audience is seated and does not pertain to offstage areas. Stage left refers specifically to the actor's left when facing the audience and does not encompass the areas behind the scenes. Backstage refers to the entire area behind the stage, including wings, the green room, and storage spaces, but it is a broader term that does not specifically denote the concealed side areas. Thus, "wings" is the correct answer, as it specifically denotes the hidden areas on the sides of the stage used for various purposes during a performance.