The Free On-line Dictionary of Computing (30 December 2018):

system unit

    The main body of a computer, consisting of a
   plastic or metal enclosure, the motherboard, and (typically)
   internal disk drives, a power supply, cooling fans, and
   whatever circuit boards plugged into the mother board, such as
   a video card.

   The system unit is occasionally referred to as the CPU,
   though this really means central processing unit.

   (2000-08-10)