Ceremonial garment (5)
(Other definitions for apron that I've seen before include "Kitchen garment or airfield surface." , "stage perhaps" , "area in front of aircraft hangar" , "Protective garment worn at front" , "Protective garment - part of stage" .)
(Other definitions for gown that I've seen before include "Loose dress or robe" , "Formal robe" , "Academic (or evening) wear" , "Elegant dress" , "Ball dress" .)