The Green Room

According to a definition given at Wikipedia, the green room at a theatre is a room or other public venue for the accommodation of performers or speakers when not required on the stage.

Its function is as a break/touch-up lounge so that performers do not have to go back to wardrobe/dressing rooms and are still easily accessible for their call.

The Wikipedia site also notes that the first recorded use of the term was in 1701 but the origin of the term is unknown and is the source of many folk etymologies including:

The term green room can be traced back to the East End of London, UK. In Cockney rhyming slang, greengage is stage, therefore greengage room is stage room and like most rhyming slang it gets shortened, hence green' room. (Attributed to comedian and dancer Max Wall).
In some explanations, it is said that the color was a response to limelight—early stage lighting, although this is unlikely as limelight was invented in 1820 and the term “green-room” dates back to at least 1701 (mentioned in Colley Cibber’s Love Makes a Man), and probably earlier.
Green is also thought to be a calming and soothing color.
The most widely accepted origin of the term dates back to Shakespearean theatre. At that time actors would prepare for their performances in a room filled with plants and shrubs as it was believed that the moisture in the topiary was beneficial to the actors’ voices.
Richard Southern, in his studies of Medieval theatre in the round, states that the acting area was The Green. The central space, often grass-covered, was used by the actors, while the surrounding space and circular banks were occupied by the spectators. Since then The Green has been a traditional actor’s term for the stage. Even in proscenium arch theatres there was a tradition that a green stage cloth should be used for a tragedy. The green room is thus the room on the way to the green. Technical staff at some West End theatres (such as the London Coliseum) still refer to the stage as the green.
In Shakespeare’s day, the actors waited in a “tiring house” (see Midsummer Night’s Dream, where Peter Quince and his troupe rehearse in the woods: “This green plot shall be our stage, this hawthorn brake our tiring-house.”) As scenery was also stored in the tiring-house, “Green Room” may be a corruption of “scene room,” the room where scenery was stored which doubled as the actors’ waiting room. Samuel Pepys mentions the Scene-room at the Drury Lane Theatre Royal in 1667. By this time the Tiring House was the men’s dressing room, the Shift was the women’'s dressing room (not needed in Shakespeare’s time when all actors were men), and the Scene Room was where the actors met, ran lines and rehearsed. “...she took us up into the Tireing-rooms and to the women’s Shift, where Nell was dressing herself and...then below into the Scene-room, and...here I read the Qu’s (cues) to Knepp while she answered me, through all her part of Flora’s Figarys..”
In Blackfriars Theatre (1599), the first fully enclosed theatre in England, the area behind the scenes where the actors waited was painted green; the green room became a generic term for the actors’ waiting area. Shadwell's play, “A True Widow” (1678), mentions, “...this Evening, in a green Room, behind the Scenes...”
In Restoration theatres, the main, seasoned actors waited for their entrances in the wings—or sometimes even at the sides of the stage—while the minor players, usually young, less experienced “green” actors, were banished behind the scenes. Hence, the backstage room was for the “green” players and came to be called the green-room.
In English theatres, a green floor-cloth was traditionally spread on the stage for tragedies. During the Restoration, when virtually all performances were comedies, the green floor-cloth was used in the actor's waiting room to deaden their footsteps so the sound of pacing actors would not disturb the performance. As tragedies were rarely performed, the green floor cloth became a fixture of the actors’ lounge and the room became known as the Green-room.

For more folklore on The Green Room, go to: http://en.wikipedia.org/wiki/Green_room