Process Capability:

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

We are often required to compare the output of a stable process with the process specifications and make a statement about how well the process meets specification. To do this we compare the natural variability of a stable process with the process specification limits.
A capable process is one where almost all the measurements fall inside the specification limits. This can be represented pictorially by the plot below:

Most capability indices estimates are valid only if the sample size used is 'large enough'. Large enough is generally thought to be about 50 independent data values.
Process Capability is given by,
Cp = USL - LSL
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6 Sigma