1. A type of screw that is used in automotive circles as a means of having an adjustable length that can be used to tune aspects of a car's behaviour. It looks roughly like this:
   ^--- screw   ^-- hexagonal  ^--- screw 
        thread      prism           thread

Note that the screw threads are fixed to the hexagonal prism (and also that one of the threads is backwards relative to the other one, so you can still screw it in to two opposite facing screw holes). The idea is that you put each of the screw threads into holes, and turn the hexagonal bit in the middle with a wrench, then you can adjust the length easily.

For example, consider the setup below:

      V-- wheel
     |  |   +-- drive shaft
     |  |   V   +-----------+
     |  |=======|           |
     |  |       |           |<--- chassis
     |  |--<=>--|
      --  ^-- turnbuckle
The above top view shows the chassis of your car. If you twist the turnbuckle one way, it will angle your wheel in, the other way, it will angle the wheel out.

It is typically found in the suspension of your vehicle to adjust things like toe in and camber. 2. In wrestling/boxing, the thing that joins the corners of the ring (which is of course, ironically, rectangular).