What exactly is the afterbody of an aircraft?
Each vessel, whether land, sea, or air, should (might) have a defined afterbody. The afterbody is defined as:


  The afterpart of a vehicle; The part of a vessel abaft midships; A companion body that trails a satellite or spacecraft; A section or piece of a launch vehicle, rocket, or spacecraft that enters the atmosphere unprotected behind the nose cone or other body that is protected for entry


and Merriam-Webster Online defines it as:


  :  the after part of a body: such as
  a :  the part of a ship abaft midships
  b :  the bottom portion of a seaplane hull or float aft of the main step  


To show you, visually, an easy to see set of examples, consider the following pictures of floatplanes (seaplanes):





In the first and second images, the afterbody is that part of the aircraft behind the step, which is the little 'notch' or, well, 'step' just behind the main wheels. In the third picture, the afterbody is everything after the step, which is the more steeply angled section under the wings. The step is generally just aft of the center of gravity of the plane.

Generally speaking, if one were to discuss the afterbody of a large airliners, such as the Airbus A380, the afterbody would be the fuselage from the wings back, but not including the tail (where the rudder and elevators are.) I could not find any Airbus references to 'afterbody' however. I did find some references to the afterbody of a Boeing 787, being that part of the plane behind the pressurized cabin, which is the very back of the plane, including the tail.

Not a very good solid definition where everyone agrees, sorry.