Body in White refers to all the sheet metal components which form the basic shell of an automobile. It is representative word which comes form the color of Steel or Aluminum which is the major sheet metal component of the structure.
Body in White or BIW refers to the stage in automotive design or automobile manufacturing in which the car body sheet metal (including doors, hoods, and deck lids) has been assembled or designed but before the components (chassis, motor) and trim (windshields, seats, upholstery, electronics, etc.) have been added.

The name is derived from manufacturing practice before steel monocoque bodies, sometimes trademarked unibody. When most cars were made by firms as just a frame, with an engine, suspension, and fenders attached, the manufacturers built or purchased wooden bodies (with thin, non-structural metal sheets on the outside) to bolt onto the frame. The bodies really were painted white as a preliminary to being painted the customer's chosen color.