Answer
The abdominal aorta begins at aortic hiatus--the discontinuity in the diaphragm through which the descending aorta passes.
Work Step by Step
The section of the descending aorta below the diaphragm is arbitrarily designated the abdominal aorta. Above the aortic discontinuity, from about T12 to T4, that segment of the descending aorta is called (arbitrarily) the thoracic aorta.
