A safety critical system is one that must function correctly to avoid human injury, human death, damage to property, financial loss, damage to the natural environment, or devastating systemic effects (such as a catastrophic drop in stock market prices). A system is judged to be safety critical when its use involves risk (a potential that a mishap could occur, with severe consequences). Most safety-critical systems are designed to assure the safe use of systems involving a hazard, a state or condition in which unsafe use of the system will inevitably result in a mishap; for example, a train moving at high speed poses a hazard. Most hazards are caused by the use of potentially dangerous or lethal amounts of energy, such as the potential kinetic energy of a train moving at high speed. See availability, fault tolerance, hazard, mishap, risk.

Safety-critical systems, also called life-critical systems, are computer systems that can result in injury or loss of life if it fails or malfunctions. These systems can also cause harm to other equipment or the environment in the event of failure.