Ward 47 was a ward of the Tilonus Institute for Mental Disorders. It was also a ward in the facility his character from the play Frame of Mind was in.
Upon awakening from his supposed delusions, Doctor Syrus informed William T. Riker that he was in this ward. Later, believing he was back in his quarters aboard USS Enterprise-D, he mentioned having awoken in ward 47 to Deanna Troi. (TNG: "Frame of Mind")