(1) The process of determining whether a product or service is adequate to perform its intended function satisfactorily. Validation is broader in scope, and may involve a greater element of judgment, than verification. Computer System Validation: The process of testing and evaluation of the integrated computer system (hardware and software) to ensure compliance with the functional, performance and interface requirements. Model Validation: The process of determining whether a model is an adequate representation of the real system being modelled, by comparing the predictions of the model with observations of the real system. Normally contrasted with model verification, although verification will often be a part of the broader process of validation. There is some controversy about the extent to which model validation can be achieved, particularly in relation to modelling the long term migration of radionuclides from radioactive waste in repositories. System Code Validation: Assessment of the accuracy of values predicted by the system code to occur. (2) Confirmation by means of objective evidence that the requirements for a specific intended purpose and use or application have been fulfilled. See: Verification. The corresponding status is termed "validated". The conditions of use for validation purposes may be real or simulated