A SIS is a Safety Instrumented System. It is designed to prevent or mitigate hazardous events by taking the process to a safe state when predetermined conditions are violated. A SIS is composed of a combination of logic solver(s), sensor(s), and final element(s).
Other common terms for SISs are safety interlock systems, emergency shutdown systems (ESD), and safety shutdown systems (SSD). A SIS can be one or more Safety Instrumented Functions (SIF).