Definition
The inherent capability of a stable process to consistently produce conforming results within a given specification range, considering the spread, center, and customer specifications of the process distribution.
In detail
Process capability is a management judgment; indices such as Cpk and Cp are just methods to quantify it. Before calculating capability, you must confirm that the measurement system is trustworthy, the process is in statistical control, and correct specification limits are used; otherwise, impressive indices are meaningless. Cp only compares spread with specification width, while Cpk also considers center offset; therefore, "process capability" cannot be directly used as a synonym for Cpk, nor can control limits replace specification limits.
Example in practice
All samples of a certain dimension temporarily fall within specification, but the control chart shows a sustained drift in the process mean. The team first eliminates special causes and confirms gage stability, then calculates Cp and Cpk. The results show that the spread is sufficient but the center is biased toward the upper limit, so the team adjusts the center rather than purchasing more precise equipment, and then re-verifies capability with new stable data.