I've worked with computers a lot, and it's generally referred to (as @FumbleFingers said) as:
- system unit
- system box
- PC or more colloquially PC box
- or just simply machine
To what @Liamdev361 said: yes sometimes it can be called a tower, but strictly
tower is just one form factor a computer can come in, the others being minitower, pizza-box, mini (e.g. Mac Mini), AIO (all-in-one), cylinder (2013 MacPro)...
To what @DavidM said: it is not the CPU. The CPU, motherboard (and many other things) are inside the system box, but isn't the CPU.