Process stability

If all factors of variation remain the same, then the amount of variation can be measured.

it means the process is varying, but in a consistent; predictable, and measurable way.

Process instability

Process instability is where exceptional things that might occur on a frequent basis such as a new piece of equipment, new operator, or change of raw materials supplier.

These changes are not part of the on-going, normal common causes, but are 'special causes' and they change the amount of process variability, negatively or positively.

When a special causes arises, the variation changes, meaning the process is no longer behaving in a predictable way so it is an 'unstable process'.