(Human Machine Interface) The user interface in a manufacturing or process control system. It provides a graphics-based visualization of an industrial control and monitoring system. Previously called an "MMI" (man machine interface), an HMI typically resides in an office-based Windows computer that communicates with a specialized computer in the plant such as a programmable automation controller (PAC), programmable logic controller (PLC) or distributed control system (DCS). Contrast with OI. See PAC, PLC, DCS, GUI and user interface.