A Safety Case is a structured argument, supported by evidence, intended to justify that a system is acceptably safe. A safety case aims to show that specific safety claims are substantiated and, in the UK, that risks are kept As Low As Reasonably Practicable(ALARP). In US, FDA issued a guidance document in 2010 to require infusion pump manufacturers to submit safety cases as part of the 510(1)s. http://www.fda.gov/medicalDevices/DeviceRegulationandGuidance/GuidanceDocuments/ucm206153.htm. Background summary about US FDA on Safety Case can be found at http://gessnet.com/index-news.html
A definition by UK Defence Standard 00-56 Issue 4 states: Such an evidence-based approach can be contrasted with a prescriptive approach to safety certification, which require safety to be justified using a prescribed process. Such standards typically do not explicitly require an explicit argument for safety and instead rest on the assumption that following the prescribed process will generate the required evidence for safety. Many UK standards are non-prescriptive and call for an argument-based approach to justify safety, hence why a safety case is required.
Safety cases are typically documented in both textual and graphical notations, e.g. using the Goal Structuring Notation (GSN).
The review of safety cases is an important activity in the safety engineering process, performed throughout development, operation and maintenance, in the which the safety case argument and evidence are scrutinized and challenged.