Give me a good definition of Process Capability…

Mike, how about this one. A little longer that the previous suggestion, but more technically accurate.
Process capability compares the range, or natural tolerance (NT), of an in-control or consistent process to the specifications. The natural tolerance can be described by thinking of the bell–shaped distribution. The natural tolerance would be the +/- 3ó range that includes 99.73% of the data. If the natural tolerance is less than the specification range, then the process is said to be capable of meeting requirements. The tolerance is simply the upper specification limit minus the lower specification limit. A process can either be capable or not. Even though a process may be capable, it may not be meeting requirements.
Paul

I like Paul definition. But can I know further how you calculate the natural tolerance? If USL minus the LSL is your natural tolerance, what is the spec range? I just want to know how we define the process is capable through this NT. Previously, I just take the Cpk or Ppk as long as it less than 1.33, I consider it’s not capable enough. Please advice. I’m under training BB trying to get certification. I’ll appreciate your help. TQ

Paul stated it very well.  Process Capability, when stable, is the random variation around some centerpoint that the process actually is.  Spec limits is usually something the customer establishes.  Whether are not the process is within those spec limits is another thing.  If they are not, you need to get them there, or the customer goes somewhere else.