The user interface (or Human Machine Interface) is the aggregate of means by which people (the users) interact with a particular machine, device, computer program or other complex tool (the system).

The user interface provides means of:

• Input, allowing the users to manipulate a system
• Output, allowing the system to produce the effects of the users' manipulation.

Introduction

To work with a system, the users need to be able to control the system and assess the state of the system. For example, when driving an automobile, the driver uses the steering wheel to control the direction of the vehicle, and the accelerator pedal, brake pedal and gearstick to control the speed of the vehicle. The driver perceives the position of the vehicle by looking through the windscreen and exact speed of the vehicle by reading the speedometer. The user interface of the automobile is on the whole composed of the instruments the driver can use to accomplish the tasks of driving and maintaining the automobile.

The term user interface is often used in the context of computer systems and electronic devices. The user interface of a mechanical system, a vehicle or an industrial installation is sometimes referred to as the human-machine interface (HMI). Yet another term used is "operator-interface console" (OIC).

HMI is a modification of the original term MMI (Man-Machine Interface). In practice, the abbreviation "MMI" is still frequently used although some who still use the term may claim that MMI stands for something different now (e.g. "Management and Manufacturing Information" or "Mammal-Machine Interface"), in order to avoid controversy.

Whether it is called MMI or HMI, the terms refer to the 'layer' that separates a human that is operating a machine from the machine itself.

In science fiction, HMI or MMI is sometimes used to refer to what is better described as direct neural interface. However, this latter usage is seeing increasing application in the use of (medical) prostheses—the artificial extension that replaces a missing body part (e.g., cochlear implants).

The system may expose several user interfaces to serve different kinds of users. For example, a computerized library database might provide two user interfaces, one for library patrons (limited set of functions, optimized for ease of use) and the other for library personnel (wide set of functions, optimized for efficiency).

In some circumstance computers might observe the user, and react according to their actions without specific commands. A means of tracking parts of the body is required, and sensors noting the position of the head, direction of gaze and so on have been used experimentally. This is particularly relevant to immersive interfaces.