The means of communication between a human user and a computer system, referring in particular to the use of input/output devices with supporting software. Devices of increasing sophistication are becoming available to mediate the human-computer interaction. These include graphics devices, touch-sensitive devices, and voice-input devices. They have to be configured in a way that will facilitate an efficient and desirable interaction between a person and the computer. Artificial intelligence techniques of knowledge representation may be used to model the user of a computer system, and so offer the opportunity to give personalized advice on its use. The design of the machine interface may incorporate expert system techniques to offer powerful knowledge-based computing to the user.

HCI is a branch of the science of ergonomics, and is concerned especially with the relationship between workstations and their operators. The aim is to develop acceptable standards for such aspects as display resolution, use of colour, and navigation around an application.

The terms human-system interface (HSI), human-machine interface (HMI), and man-machine interface (MMI) are all used as synonyms.