Source : Free On-Line Dictionary of Computing

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)