What is process capability?

It is an ability of a process to meet customer's specifications without generating defects beyond acceptable limits, through the evaluation of the spread and centering of the process.

This study is done by evaluating

"How precise the measured data from the lot?" (Cp) - Done by studying how the close the data points are towards the average - Spread of the process.

and

"How accurate the measured data from the lot?" (CpK) - Done by studying how close the average towards the centre - Closer to the target, higher the CpK as more area of the curve false within the spec limits.

A: The variability allowed by the specification limits. I believe this is measured as the Cp number or how well the process fits into specification (spec) limits. The total variation allowed is simply the upper spec limit minus the lower spec limit (USL - LSL) B: The process can meet the product spec Refered to as the Cpk, or how well the process "fits" between the spec limits and is centered between them.

Process capability compares the output of an in-control process to the specification limits by using capability indices.

It can be described as the capability of a process to meet its purpose.

In enigneering/production the process capability indicates how the output of  the process meets the  requirements (for example engineering tolerances, customer requirements, etc.)

For IT processes ISO15504 describes a measurement framework to assess process capability.