Special Cause Variation

A type of variation (a.k.a. Signal or Anomaly) that reflects unnatural or erratic variability that generally comes from outside the process.  Some examples include variation caused by a power surge, extreme weather conditions, system/computer malfunctions, poor batch of raw materials, etc. This can be contrasted with Common Cause Variation.
Related StatStuff Video: 
   -Section 5-Measure Phase, Lesson #14 - Variation Causes (Common vs. Special)