The size of the hole in the back of the wheel that centres the wheel properly on the hub of the vehicle.
Many aftermarket wheels will be drilled to a larger universal size, and use hub rings (spigot rings) to reduce the size to match the hub on the car for perfect centering. Generally speaking, a perfect hub size match and hub rings can be optional for vehicles that use lug nuts, but are absolutely required for vehicles that use bolts such as BMW, Mercedes, and VW and most European vehicles.
Use of a hub centric ring can help in reducing minor vibrations. Proper installation and torque sequence is required to correctly centre a wheel with or without a hub centric ring (spigot ring).