A waiting room was a room in which patients awaited medical care.
According to Quark, the Ferengi Doctor Orpax charged patients two strips of latinum just to walk in to his waiting room. (DS9: "Body Parts")
Doc Brown used to have lollipops in his waiting room. (VOY: "Cathexis")
In 2371, Tuvok suggested that an Earth-like environment created by the Caretaker was essentially a waiting room. (VOY: "Caretaker")