In Statistical Process Control, the variation in a process is categorized as either Common (General) Cause Variation or Special (Assignable) Cause Variation.
The common cause variation arises from a multitude of small factors that invariably effect any process and will conform to a normal distribution, or a distribution that is closely related to the normal distribution. The special cause variation arises from specific factors that have an identifiable effect on the process.
Common cause variation is inherent in the process and can only be reduced by changes to the system, and usually requires management action eg. upgrading a machine. Special causes can often be tracked down and fixed without extensive changes to the system eg. fixing a fault on a machine or improving the quality of raw materials.