Overview
Functional safety is, simply put, “Protecting a user from technology”. It also protects technology from users. More technically however, the definition of Functional Safety is, “Systems that lead to the freedom from unacceptable risk of injury or damage to the health of people by the proper implementation of one or more automatic protection functions (often called safety functions). A safety system consists of one or more safety functions.”
TUV Functional Safety Overview
Functional Safety -- Wikipedia entry