Systems of hardware and software which are designed to detect hazardous out of control conditions and return them to a safe state. By definition they must be automatic and independent of other protective or control systems.
A separate and independent combination of sensors, logic solvers, final elements, and support systems that are designed and managed to achieve a specified Safety Integrity Level (SIL). An SIS may implement one or more Safety Instrumented Functions (SIFs).
Reference: Interlock System definition by Guidelines for Initiating Events and Independent Protection Layers in Layer of Protection Analysis, CCPS, ISBN 978-0-4703-4385-2