A waiting room was a room in which patients awaited an appointment or consultation with an individual, particularly for 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")