This is a work in progress, this article is by no means complete yet!

Guideline on General Principles of Process Validation

Center for Drugs and Biologics
Center for Devices and Radiological Health
Food and Drug Administration
May 1987

This is the initial guideline and is included here for completeness and those taking an interest in it.
In 2011 this guideline was superceded by Guidance for Industry - Process Validation: General Principles and Practices as part of the "paradigm change".
This guideline leans heavily on 21 CFR Part 820.

Of special interest are:
The FDA defines process validation as follows:
Process validation is establishing documented evidence which provides a high degree of assurance that a specific process will consistently produce a product meeting its pre-determined specifications and quality characteristics.

Get it here!