Safety Instrumented System definition in Computer Security terms:
Definition(s): A system that is composed of sensors, logic solvers, and final control elements whose purpose is to take the process to a safe state when predetermined conditions are violated. Other terms commonly used include emergency shutdown system (ESS), safety shutdown system (SSD), and safety interlock system (SIS).
Source(s): NIST SP 800-82 Rev. 2 (ANSI/ISA-84.00.01)
reference: CSRC Glossary