variation attributable to a large number of small sources of variation. Added together they may result in a high level of variation in a process and/or a large amount of waste. The sum of these common causes determines the inherent variation of the process and its capability (as defined by control limits).
variation that is not attributable to the process all the time. Special causes are special circumstances.
Shewhart showed that a stable process is one that shows no special-cause variation. Although it’s behaviour may be random, it will perform predictably – you can expect any performance between the upper and lower control limits. This defines the process capability.