Process Validation is establishing evidence which provides a high degree of assurance that a specific process (such as the manufacture of pharmaceutical dosage forms) will consistently produce a product meeting its predetermined specifications and quality characterisitics.
Hi,
I would like to add the following: 
PROSPECTIVE PROCESS VALIDATION: Validation conducted prior to the distribution of either a new product, or product made under a revised manufacturing process, where the revisions may affect the product’s characteristics.
RETROSPECTIVE PROCESS VALIDATION: Validation of a process for a product already in distribution based upon accumulated production, testing and control data.
CONCURRENT PROCESS VALIDATION: Establishing a documented evidence that a process does what it purports to do based on information generated during actual implementation of process. Concurrent validation is performed when the frequency of manufacture is insufficient to satisfy prospective validation requirements.Batches may be individually released provided there should be sufficient assurance that each batch was thoroughly monitored and tested.
Nice addition