UX Glossary · H
Human-machine interface
Also: HMI · MMI
The controls and displays through which a person operates a machine, especially where mistakes are physical.
A human-machine interface is the boundary through which a person monitors and controls a machine: instrument clusters, control-room screens, medical device panels, industrial terminals. It covers physical controls as well as screens.
Why it is not just UI with different scenery
Three constraints separate it from most software design.
The user cannot opt out. A driver cannot close the app. Whatever the cluster displays, whenever it displays it, is what they get, often at speed and with divided attention.
Consequences are physical. An ambiguous state in a spreadsheet is annoying. An ambiguous state in a vehicle or an infusion pump is a safety issue, which is why HMI work is specified rather than iterated in production.
The environment is hostile. Vibration, gloves, glare, noise, temperature, interruption. Legibility requirements come from the context, not from taste.
How the work differs
Specification carries far more weight. Flowcharts, state diagrams and precise interaction descriptions are the deliverable, because a supplier builds from them and an ambiguity becomes a wrong behaviour in a shipped vehicle.
Validation leans on simulators and controlled studies rather than remote sessions, and standards including the ISO 9241 series and domain-specific regulation set hard requirements.
In practice
The discipline HMI work teaches, and that transfers well to ordinary software, is that system state must always be stated rather than implied.