Systems and processes represent the methods people use to meet requirements. Systems are either stable and thus predictable or unstable. Stable systems include only common cause variation and unstable systems include both special and common causes. For example, if you always get to work or school on time and can predict the range of your arrival time, the process probably represents a stable commuting system. However, if periodically, you are unpredictably late for one reason or another, this would represent an unstable system.
(SuccessThroughQuality.com Glossary)