(1) a statistical measure of the inherent process variability for a given characteristic.
(2) a measurement of the variability in a process to some specification criteria.
(3) the number of standard deviations that will fit between the target value and the specification limit, i.E. A process with 6 sigma capability can fit 6 standard deviations between the target and the upper or lower specification limit. A higher value indicates a more capable process.
A visual animation: Process Capability - Cp vs. Cpk
Elsmar Cove forum Process Capability discussion threads.