What does PCD mean?

PCD or Pitch Circle Diameter refers to the bolt pattern on a vehicles wheel hub.
As an example, Volkswagen have many models that use a 5 x 112 PCD.

So what does this mean?
The "5" refers to the number of bolts or studs used to secure the wheel to the hub. The "112" refers to the diameter of the circle in mm. The bolt holes are positioned equally spaced in a circular pattern.

If you drew a circle through the centre of each bolt hole, the measurement across the centre of the circle from one side to the other (the circle diameter) in our example would be 112mm.